I'm sorry to hear you are having trouble with Ninjaboard Beta.
There are several possibilities:
-Perhaps you didn't create content (forums) and a setting for Ninjaboard yet.
There's sample content available for Ninjaboard which gets you started. You can install this via Components > Ninjaboard > Tools > Sample Content (button).
-May be you are using a third party SEF extension which is causing 404 pages for Ninjaboard. Try to exclude Ninjaboard from there.
- Uninstall a previous BETA version of Ninjaboard and re-install the new BETA version.
If this doesn't help, may be you could give access to your site and I will try to see what the problem could be.
I hope this helps.
Let us know how it goes.