Well, go to
www.toontown.com (Sorry, Mods for posting a site) and scroll down till you see
Test Server Now Accepting New Toons - Apply Now! then click that. Then, if you don't have a Test Toontown account, sign up. Just put in your User name and password to the plain Toontown and it will take you to were you sign up for Test Toontown.
Hope this helps.
-MLHM-