Turn on Home Sharing in iTunes (Advanced menu) on your computer and on your iPad (Settings > Music/Video and enter your Apple ID). Next, launch the Music or Video app on your iPad. In the Music App, tap the "More" button at the bottom, find the little house with "Shared" next to it. Tap on that and then on one of the Shared Libraries. It might take a few seconds to update, but then you can listen to all of that Library's media on your iPad. For video, tap the Shared tab at the top of the screen. Enjoy!