If your using the desktop version of slingplayer, you can find the Id by doing the following:
Open slingplayer, click on help, about your slingbox, information and it will propogate a page with information including the ID # your looking for.
If your using the web player, then go to :
Once it connects, click on "options" and you'll get a pop up with the information your looking for.
The following links will provide you with all the information about how the Slingbox ID works and where to get it.