The ID might be listed here. If not:
What model of Slingbox?
Slingbox 350, M1, and SlingTV/Slingbox 500 do not have user-facing Slingbox IDs.
What SlingPlayer software(s) have you used to watch the Slingbox?
Just taking a guess--if the SlingPlayer software used is old enough or exclusively the legacy SlingPlayer Desktop for Mac 1.0.10, the only time the requestor may have seen the Slingbox ID was at setup time (i.e. no Sling Accout was ever created and there's no link between the Slingbox ID and a Sling Account, thus the "How to find your Slingbox ID with SlingPlayer" portion of this article).
What SlingPlayer software are you using on your daughter's Macbook?
AFAIK, the only SlingPlayer software on the Mac that requires a Slingbox ID is the legacy SlingPlayer Desktop for Mac 1.0.10 (the rest of the software (Watch on Slingbox.com and Slingplayer for Desktop for Mac) operate on Sling Accounts).