Being an Endpoint customer doesn't entitle you to use the Web Gateway, but I'm guessing you are probably a Protection Suite Enterprise Edition customer which does entitle you to a Web Gateway base license and other products such as the Messaging Gateway and also Mail Security for Exchange and Domino products.  If you are looking to test the URL Filtering add on for the Web Gateway I think your best bet is to just register for Trialware on the symantec.com Web Gateway site below, when you register there you should get sent a 30 day Web Gateway license which includes the url filter.  Once you install this license you should then be able to enable the Content Filter module to conclude your testing.

http://www.symantec.com/business/web-gateway

Hope that helps!

Kevin