That is typically caused from a Jquery conflict. http://codex.simple-press.com/codex/faq/troubleshooting/what-is-this-jquery-conflict/

Hard to say though without actually looking at it and trying it. Can you supply a link to your forum?

actually my forum is not ready yet, I’ve just installed and now I’m customazing it.

Problem is that I’m trying to creat a topic for test purpose only but nothing happens.

I can see this message at the bottom “javascript:void(null)” when I click on creat topic.

I also found this..

http://codex.simple-press.com/codex/faq/troubleshooting/i-am-seeing-javascriptvoidnull-for-a-browser-link/

any help? 

hard to say without seeing it… but likely a js conflict…

start with this: http://codex.simple-press.com/codex/faq/troubleshooting/what-is-this-jquery-conflict/

and if you can give us a link we can look for obvious ones…