It seems to be available and working for me now, maybe it was just down for a bit?
Do you have a My Digikey account? If so you should be able to access the BOM upload by doing the following:
Go to Resources, BOM Manager
Click the Login link down at the bottom of the page, and login with your My Digikey account (or register for one).
In the Quote/Ordering box, click the Parts List / BOM Manager link.
In the BOMs / Kits box, click Create New Bom (Upload File), then upload your Excel BOM file.
Hope this works for you now!