Samantha, click on "User CP" and when that loads, click on "Edit Options" which will be on the left.
Under the "Messaging & Notification" area, you will find in the second section your options for notification and subscription called "Default Thread Subscription Mode". There is a drop-down box. The options are:
Do Not Subscribe - post on threads without automatic subscription
No Email Notification - auto subscribe on all threads you post on without email notification when anyone posts to them
Instant Email Notification - all posts to your subscribed threads by anybody are sent to you via email at that moment
Daily Email Notification - You will be notified via email once daily of all threads you subscribed to that are posted to
Weekly Email Notification - you get the idea.
I choose the second, since I'm on here every day anyway and I can always unsubscribe from any thread I put a one-time post on.
You are probably set at "Do Not Subscribe." If that is the case, you will not be able to subscribe. Try changing your option on your control panel and see if that does the trick. If not, post a help thread in the Support Forum and admin will see to it. Remember to save your changes.
Happy subscribing!!