In order to be able to create a Wiki, the repository administrator must have enabled this feature. The step by step instructions are available on the Enable a Wiki page.
After this feature is enabled, the Wiki link will be displayed in the left panel. When you click it, the Wiki welcome Home page appears by default.
On the top right corner of the Home (and each wiki page), you'll find the "Create page" button to add new pages.
You can also have a look at the Wikis page for all the related information.
