It could be a theme or plugin issue
Plugins
Deactivate all but bbpress and see if this fixes. if it does, re-enable one at a time to see which is causing the error.
Themes
If plugins don’t pinpoint the problem, as a test switch to a default theme such as twentytwelve, and see if this fixes.
I have tried both two ways, but I cannot see Forum in Dashboard. I wonder if it is needed any extra step.
I have tried again, and it works. thank you so much. But it also points out that bbpress conflicts with one of my plugins. Is this true?
probably, which plugin does it conflict with ?
I think it is wpml. when I deactivate this plugin, Forum works. One more thing I want to ask is that what is the difference between forum types (Forum and Category). I create a new forum and I choose Forum type. then I create another new one with Category type. I don’t see the difference between appearances of these two forums.
There seem tio be issues with wpml and bbpress see
http://wpml.org/forums/topic/wpml-plugins-conflict-with-bbpress/
as an example.
I’d suggest you try raising a ticket with wpml to see fi there has been a resolution since that post
