Templets I’m so lost

Oi!

I found a theme I’d really like to use. I have it uploaded to the right folder and when I logged in to change themes it said:

“The following themes are installed but incomplete. Themes must have a stylesheet and a template.”

Ack! The .css is there, but no template? I don’t know anything about this template stuff. Should the theme have come with one? I got it from

http://www.alexking.org/software/wordpress/style_browser.php
(Diary, to be specific)

It said something about a switcher, but it looks like I can switch okay. I just need a template.

Am I going to need hours and hours of coding to get this to work? Can someone just point me in the direction I need to go? Pweese?