Go to the hardware store and get a couple of P-clips in the same diameter as your seatstay* tubes. Mount the P-clips to the frame, and the rack to the P-clips.
Thanks for the info. My current beater MTB has some holes near the rear wheel where I can attach, and I assume the Denali has the same thing, though it's hard to see in the picture. I'll have to see if I can download some more detailed specs about the bike - there's not a lot on the Kent website.