It sounds like you may be using the desktop software to set up the Slingbox. In order to set up a Slingbox 350, you would want to use the Slingbox website www.slingbox.com. Once on the site, you will click on watch, and then in the directory, you will see either a link that says setup a new Slingbox, or a Slingbox called "My Slingbox" with a yellow dot which means it needs to be configured. It will then walk you through the setup process. Setting the Slingbox up on the website will not ask for the 32 character Slingbox ID. This Slingbox ID is only available once the Slingbox is set up and is only used for the Slingbox PRO-HD and older.
Hope this helps,
Sling Media Support