The only music that can be downloaded from an iPad to iTunes on a computer is music you purchased in iTunes.
That music would be downloaded by plugging your iPad into iTunes, and doing File > Device > Transfer Purchases from iTunes' menu.
The rest of your music (if any) needs to come from a computer backup or other source (e.g., CDs).
I cannot get my music off the Ipad into the itunes libary on my new computer.
1) From your OLD computer...
Copy your ENTIRE iTunes FOLDER to an External Drive... and then from the External Drive to your New Computer..
Full Details Here > http://support.apple.com/kb/HT1751
An Added Bonus is that you will have a Backup of iTunes.
2) If you do not have access to the iTunes Library on your Old computer... or its Backup...
See these 2 Links...
Recovering your iTunes library from your iPod or iOS device
Syncing to a New Computer...