Yeah, there are a few complicated ones, but this one was pretty simple. As long as you have Photoshop to open the template file, the rest it's just replacing the text information for your own and add your photo.
Yeah, that's the problem about templates. Because we need to save all layers without merging them (flat them into one pic) to create a template that can be altered by whoever uses it, the only way to do that is to save the file as the software we are using to make it, no matter if it's a Photoshop file, a Gimp file or any or art software.
No, you don't need to start a new layer. Just go to "File < "Open..." and the select the directory where you downloaded the psd file into. It should open nicely. Then it just select the layers that you want to change, flat them after you are done, and save it as jpg or png.