Hi, Geoff! I know EXACTLY how you feel but I was able to install the Mystic River template on my site and it looks gorgeous (except for the header spacing that I haven't worked with yet).
Have you downloaded and unzipped the template file?
Think of the unzipped template as the result of a big trip to the grocery store. You need to put the teabags with the teabags, the salt with the other spices, and the milk in the door of the fridge. But since you've been fed intravenously all your life <g>, the files come with labels to tell you where to put them away. (Hey, it's the middle of the night and I'm sick, I make no apologies for weak analogies.)
If you compare unzipped files with the structure of your test or live ZenCart, you'll see some similarities. (Everyone smart will tell you to always practice on a test store first.) Both will have an includes folder. (Depending on how you unzipped the template, you may have to go down through a couple of directories named something like Darkness to get to the includes directory.) Open both includes directories. Now, in the Darkness directory, you will probably see languages and templates directories. If there are any files in your Darkness directory that aren't in your store, copy them into your store. Then pick one directory and open it, both in your store and your unzipped Darkness directory. Again, compare files, upload any that aren't in your store, compare directories, upload any that aren't in your store, then pick a directory that is duplicated and repeat. Follow whatever pattern you choose to make sure you compare all the files and directories.
I have been told Filezilla can do this all automagically, but I haven't tried it myself.
Once you are done, you can go into Admin - Tools - Template Selection and click "Edit." A dropdown menu will appear that should have Darkness as an option. Select it and save. IMMEDIATELY go into Admin - Tools - Layout Boxes Controller and click on "reset" at the bottom of the page. If it worked, you can go to the store and you'll see the Darkness template. If it didn't work, go back into Admin - Tools - Template Selection and switch back to the Classic it was probably on.
I don't use EZPages so someone else will have to tell us both where they are stored and how to copy your modified pages into a Darkness directory.
Sylvia
PM at your own risk, I don't check in regularly!
MamaSylvia.com My ZenCart tutorial