Users cannot set up new topic, only admin

Hi All

This is the first time have set up forum, so learning slowly. I have set up catagories and users can reply to them, but are unable to start their own topics. Under catagories I have ensured that I didn't tick only admin can set up topics, so don't know where I going wrong. When you go to set up a topic it takes you into a page which says what you are looking for can't be found, or something similar.

Any help really appreciated.

Thanks
Kate