the ipad has to be associded to the computer and you will need sign to the itunes and aurthorzed the cpu and do a back up, after that go to the summery tab and under options select manualy manage music.
If its never been synced to this particular computer then by default that is true.
iTunes will only let you sync to one library at a time, if you get this warning message, follow this:
Syncing to a new computer without losing Data
You can do it by manually managing music and videos
To sync to your iPod go to iTunes>Help>iTunes Help>Sync your iPod....>Sync You Device>Set up Syncing>Sync your device manually and follow the instructions