Hi, description of the EndpointAddress parametr of the Silverlight web part says:
"If specified, its value is added as parameter with key 'endpoint' to the application parameters collection. You need to handle this parameter in your Silverlight application to take it effect." It means you don't have to use it to get your web service working properly. I recommend you to read my blog post about Silverlight communication methods, see http://devnet.kentico.com/Blogs/Petr-Vozak/February-2009/Data-communication-in-Silverlight-2-0.aspx . Anyway, example of using endpoint address parametr from Silverlight web part can be seen in our code examples which are installed on your computer together with the Kentico
CMS, you can find them in <kentico installation folder>/CodeSamples/Silverlight/ProductList. This Silverlight application is a simple product list, see more details here http://devnet.kentico.com/Blogs/Petr-Vozak/November-2008/Kentico-CMS-supports-Silverlight-2-0.aspx . I hope it helps you.