TWEN stands for "The West Education Network" and is a site which allows your professors to easily post classroom materials online. TWEN is divided into courses, and you must enroll yourself in the appropriate course to gain access to your professor's materials. To do this, go to http://lawschool.westlaw.com , sign in on the left, then find the TWEN link/tab at the top of the page. Click that, then on the next page click "Add A Course". You should then see a list of available courses for TU. Enroll in the appropriate class.
Note - some professors password protect their course- see your professor for the password.