If you want to edit a page in the Wiki, just login at the top right of the page.
If you don't have an account, one will be created when you login. Anyone can edit any page! If this is your first time editing the Wiki you can go and read the Wiki Guide, and you can play around in the Wiki Sand Box. You can also jump right into the action by clicking the 'edit' tab on the top left of any page.
If you edit often, check out To Do for tasks that need to be accomplished.
If you are looking for ideas for things to add, check out the daviswiki — because, well, it's awesome.
See also: Wiki Community