Normally, you will see a blue "+New" button to start a new thread on a forum:
If you would like to start a new discussion or question on a particular Forum and are unable to do so because you do not see the "+New" button, there are a few possible causes:
1) First, ensure that you have signed in to your Bentley Communities account.
2) Check if the Community you would like to post in requires membership. To see if this is the case, go the the community "Home" and see if there is a "Join..." link to the right of the community description. If there is, click that link.
3) In some rare cases, a problem with your Communities account may cause this. In particular, this can happen if the username was changed. In this case, please post on the Communities Feedback forum and a Site Administrator will assist you. If you are unable to post on the Feedback forum either, please contact Technical Support.