How to back up iPad to time capsule

You cannot backup directly.. in fact without a file app the iPad cannot even see the TC.

iPad and all iOS devices are designed to backup automatically to iTunes.. either locally or via cloud.. and if locally then Time Machine will backup the backup to the TC.

You might want to take a look at this Apple support document if you are interested in iOS backups.

http://support.apple.com/kb/ht1766