Got the same problem. I asked at the helpdesk but got no answer thus far...Annoying...
A decade underground is not the LPUX CD, it's a present with the LPUX. The LPUX CD can be bought on the LPU shop I think. I got mine at the London LPU summit, but I think we can bought it anywhere.
I didn't buy LPUX, but I thought A Decade Underground was for LPUX pre-orders only. The fact that it's not advertised on the website means it's not up for offer, i guess. ....Unless someone else can come in there and show that they got a download after November.
Get a copy of Songs From the Underground instead. It's only like two songs different.
@LPisgreat: you will not be receiving a CD or any other stuff from linkinpark.com. anything extra you want, you have to buy.