[NeDra]

No idea, but what I mean is if url rewrite is installed create a new rule that redirect
http://www.mysite.com/urlrewrite/index.html
to
http://www.mysite.com/index.html
just to test that the url rewrite mod actually work.

If it doesn't work, it just obvious that having all your url modified with this tool will make your forum unaccessible.

Options +Indexes
Options +FollowSymlinks
RewriteEngine on
RewriteBase /
RewriteRule ^urlrewrite/index.html$ /index.html

Just a simple test to make sure that the url rewrite work, if it does work, please confirm what option do you use inside the tool?
If it doesn't work, I can't really help you, google is your friend and get it to work.
As soon as it work, you can give it a shot and retry this addon.