There are two main reasons why the forum may not be allowing you to post:
1) If you are logged in and you try to post and the system asks you to log in - or forum is not specified, then you are most likely using security settings within your web browser that are too secure to allow the vBulletin system to work correctly.
If doing the above does not work, please go to your MIGWeb "user cp" and make sure that you have got cookies enabled in your settings.
2) When you post it tells you that you do not have permission to post in the specific forum, you have most likely changed your user password or email address recently.
When changing either of the above, an email is sent out to your registered email address to make sure that it is valid. This also acts as a security check. You MUST respond to this email by clicking on the link provided. You account will remain disabled until this is done.
If you have tried the above with no luck, please PM me the exact problem including you user name and password so that I can investigate further.