When at the Dell drivers support site, my service tag cannot be found by the automated method nor is it "recognized" when I type it in manually. What's up with that?
How long ago did you buy the PC?
I had this problem as well, it suddenly appeared on there a few weeks after my PC arrived; this was after communication with Technical Support though so they may have added it on for me. Best bet is wait, contact Tech, or manually enter your PC specs.