What OS are you using? If you are using Win XP, it does not recognise MTP, and therefore will not recognise the Fire HD.
oops, looks like you have Win 7...sorry, can't help. Have you tried the Device subforum in the Calibre Forum?
Last edited by GoSharks; 01-20-2013 at 12:33 AM.
Reason: Looked closer at the post!