To enable your Wiki tab, drag it to the 'Current Tabs' box on the 'Tabs' page, accessed by clicking the 'Website' tab on your management toolbar.
You may have already enabled your Wiki, and if so a Wiki tab should appear on your group's navigation bar:
If at any point you wish to disable your Wiki you can do so by clicking on the 'Wiki Settings' link in the top of right of your wiki page, below your group's navigation bar. Scroll down this page and select 'No' on the option for 'Enable the Wiki?'.
You can also create other wiki pages by clicking on the 'Create page' button either on the 'Index by Title' or 'Index by URL' pages after clicking on their links in the top right of your wiki page, below your group's navigation bar.
Read more about adding content to your wiki here.