I don't have access to a USB tablet right now to test this, so I'm going off memory here, so take with a grain of salt...
Mac OS X includes a technology called Inkwell, which allows you to use a tablet, or touchscreen if you are lucky enough, to write anywhere, into any application (it might only be Cocoa Apps).
This is built in and requires no extra software. To see some information about it, open the help viewer on your Mac, and type 'Ink' into the search field. One of the first hits will be 'writing text with your graphics tablet.'