I set my project to 'use wiki homepage' so why isn't my wiki showing up?

TeamForge currently uses two distinctly different wikis.

If there is the wiki you have already edited, which is available by clicking on the Wiki button at the top of any project page, and there is the 'project home' wiki, which is what you enabled in the Project Admin screen.
If you visit the project home page after setting this option, and if you have the proper RBAC permissions, there should be an Edit button in the bottom-right corner of the home page under the graphs. Use this button to edit the project home page wiki.