I just installed Forums plugin and I got my first forum set up on a multisite installation - installed it and set it up according to your instructions (first in Network Admin and second on the site). Created a test post, which shows up fine - but when I click on the post itself, I am taken to a page that says "Page Not Found".
Link to forum page: http://zionexpo.com/support/
Link to the post's page:
Help! Thanks much!