iDisk is part of the (soon to be discontinued) paid-for MobileMe service. It's basically an simple online storage facility.
If you don't have a full, paid-for MobileMe account, then you can't use it. Otherwise you just download the app, enter your full firstname.lastname@example.org MobileMe email address and password and log in.
iDisk is part of Apple's MobileMe subscription-based service. iDisk provides 20GB of online file storage as part of the subscription. MobileMe is being replaced by iCloud this fall and Apple is not accepting any new subscribers at this time, so if you do not already have a MobileMe account, you will not be able to sign up for the service. You'll need to wait for iCloud or investigate a different online file storage solution such as Dropbox.
If you do have a MobileMe subscription now, see this page for instructions on configuring it on the iPad: