This is not a tech support issue, but this is the best place to put this issue. The Latitude D400 that I have, Dell's website won't take the service tag or express service code and I don't know why, but it could be that I had bought it as a second device and not directly from Dell. (But Dell's website took the service codes for the Latitude D410 and D420 I have.) Service code will not be posted due to the Admin Policy.
The fact that you did not purchase from Dell probably means it is not registered. Have you called Dell tech support yet as they can help. Also check the service tag in the BIOS to make sure it matches the label on the system. Restart the system and at the Dell splash screen start tapping F2. The service tag number will be in the BIOS. The following post may give more information.