Read this, it might help you find your Sling ID:
If you're using the web player only, try the full Mac app. It should be able to find the Slingbox as long as you're connecting from your home network, and has the setup assistant built in. it can also find your Sling ID once it sees your Slingbox.
Here's a link to the Mac Player if you need it (click on Configure Download Slingplayer):
Once you have your Sling ID and your Slingbox is set up, it should be pretty easy to get the web player to work.
Hope this helps,