The first thing you're likely to notice after installing one of these templates (or, I guess, any of the other Gantry-based templates) is that, despite the fact that you have set it as the Site Default in the Template Manager (you did do that, didn't you? :-)) it doesn't seem to have changed the look of your site. This is because each page of your site has a particular template assigned to it, and they are currently all still using your old template. (I believe setting a template to "Default" simply means that any new page will use that template unless told otherwise.)
To fix this, go to the Styles page of the Template Manager and click on the name of the template in the Style column. (Clicking the similar link in the Template column takes you to a different page.) Now click on the Assignments tab. Here you should see a list of all the pages that are actually assigned to your template. Tick the ones that should be assigned -- not the Toggle Selection button, which can be quite useful! You probably want to select all the pages of your site -- but if you have the default Joomla data installed, it is probably best not to select the "Australian Parks" and "Fruit Shop" pages... Then simply hit Save. Et voila!
The second thing you're likely to notice, once you've got it working, is that you have lost your menus. This one had me scratching my head for a while, but it's actually quite simple (once you know how!)
First, go to your Module Manager. Yeah, I know, you wanted to go to your Menu Manager, but that won't help. The way it works, as far as I can tell at this stage, is that you define your menus in the Menu Manager, just as you define/create your articles in Article Manager. But to display an article, or a menu, you must place it in a Module, and then tell that module where it belongs. There are different types of Modules, and for now we're interested in getting our Main Menu back in the sidebar where it was in the last template. So, go to the Module Manager.
First we want to filter our list of modules so it's a little less scary! (Although note, when you first look at the list, it will likely only be displaying 20 items. You can page through the list, looking for the Module you want -- or you can change the "Display #" option down the bottom to display more items per page, or "All".) To filter, look for the "- Select Type -" dropdown box along the top of the module list, and pick Menu from the available options. That's better!
Now, find "Main Menu". (Bizarrely, my site has two of them listed. Not sure what's happening there!) Anyway, look along the "Main Menu" row.
Now, to fix the positioning of a module, you need to know two things: what does its old position really mean (is it top, bottom, left, right?) and what is the equivalent position in the new theme? To find this information, the simplest way is often to Preview the theme right here on your own server. First you need to enable previewing (and I suspect that, once you go live with your site, it would be a good idea to disable it again!) Go to your Template Manager and click on the Options logo across the top somewhere -- the thing that looks like a light switch. In the Templates tab there is only one option: "Preview Module Positions". Set it to Enabled and Save.
Now, back in the Template Manager, switch to the Templates tab if you weren't already there. Each of the site templates listed should now have a "Preview" link. Click that, and a version of your front page will be opened showing all the available positions, by name. They may not be entirely easy to read, but a bit of investigation should tell you that "position-7" is the sidebar, and the corresponding position in Gantry-based templates is "sidebar-a" (or "b" or "c", since their position on the left or right is configurable...)
Switching back to the Module Manager, we could just change our "Main Menu" to the new position (by clicking it -- or ticking it and selecting Edit -- and clicking on the "Select Position" item which brings up a long list of all the available positions) but chances are there will be several modules with their position set to "position-7"; we should probably change all of them at the same time. To do this, change the filtering from "Type: Menu" to "Position: position-7". Now simply select them all, and use the "Batch Processing" section down the bottom to Move all selected modules to the new position.
There is one slight catch, unfortunately: this drop-down box will only list positions that have already been used by at least one module, so "sidebar-a" will not appear in this list unless we manually set at least one module to the new position first. The other thing to watch for: I'm sure that when I did this, all the modules I batch-processed were automatically changed to "Unpublished". Don't forget to re-"Publish" them.
And that's it for now. More to come, no doubt! Now ... how do I publish this? :-)
Edit: Well, I got this on the front page -- sort of. It's not behaving quite how I expected, but I'm sure there's a way to sort that out. This will do for now, though.
Quite simply, I checked the Category (and "Featured=Yes" value) for the article which was in this spot, changed that to something else, then changed this one to match. And here it is. What I really want to do is move this whole index page away from the index position, and produce a new page more to my own specifications. But that can wait... :-)