There should be an "Edit this Page" button on the bottom of every wiki page. Dunno if you've used Wikis before, but basically, anything that is in CamelCase becomes a link to a page. If you click on one for a page that doesn't exist, you get the option to create it. There's special formatting you can use as well.
Trac's Wiki documentation should get you started. The best trick is that if there's a page with some formatting you like, just click edit to check it out.
Btw, if you don't have an "Edit this Page" button, I messed up your account permissions.