I eventually discovered a little scrollable window in the bottom right hand corner that gives quick routes to different parts of the forum and shows clearly how it's built up. Admit I was very confused before I found it!
You will notice in your posts, at the left hand side it says that your group is 'Validating'.
This is because you have not yet validated your membership in the forum, and until you do so, you will only be able to post in certain sections.
To validate
You will have provided an e-mail address when you registered as a member. Go to your e-mail inbox and you should have an e-mail from the forum. Follow the instructions in the e-mail to validate your membership, and then you will notice that your group turns to 'Member'. You will then have full access to all areas of the forum.
If you don't have an e-mail, read this link for what to do next.