Question: New users are registered, but seems they can not post.
Answer: Try the following indications.
1) Permissions
Check the following
Admin panel
permissions
groups forum permissions
registered users
the forums
standard access
2) Languages
If you're not using the default British English language you need to make sure you've upload the correct style imageset for the one you are using.
If you're using US English you can duplicate the imageset/en folder and rename it to /en_us