Any sub mod can click "sub settings" on the right bar, and here are the wiki options:

wiki enabling settings:

• disabled - Wiki is disabled for all users except mods
• mod editing - Only mods, approved wiki contributors, or those on a page's edit list may edit
• anyone - Anyone who can submit to the sub may edit

This sub has this setting set to 'anyone' so anyone can edit the wiki. Then you can set karma limits, which I've set to 0 for this sub:

• Sub karma required to edit and create wiki pages: 0

• Account age (days) required to edit and create wiki pages: 0

And that's it! If you have a sub, you automatically have a wiki.

When you're on a sub's page, you can access the wiki with the 'wiki' tab at the top of the sub, next to the other post sorting tabs like 'hot' and 'new'.