Forums is just a feature that plugs into mojoPortal, it does not have its own user profile because mojoPortal already has a user profile. Click the "My Account" link to get to your user profile where you can edit your signature and other profile items. The signature is displayed in the forums to the left of a user post along with their avatar or gravatar and an indicator or total posts by the user.
We don't currently have anything in the profile to subscribe to your own posts, but the forum does allow you to subscribe to either an entire forum, ie you can subscribe to the developer forum or the skinning forum or the Bug Report forum or all of them if you want to from the main forums page, there are links along the left side for subscribing to individual forums.
Alternatively, you can also subscribe to individual threads, when you start a thread or reply on a thread, if you are not already subscribed to the forum then it provides a checkbox where you can subscribe to the thread when you make a post.
Hope that clears it up.