I just tried it on my iPhone, but it should be similar for iPad.
Download the Remote application from the iTunes/App Store.
Open it and enable Home Sharing, enter your Apple ID credentials. Open iTunes on your Mac, select Advanced » Turn On Home Sharing. Enter your Apple ID credentials there.
Now your iOS device can access your Mac's library and you can remotely control iTunes from your iOS device by using Remote. You can play the audio via the computer's speakers, or via any connected AirTunes speaker. Which the iPad is sadly not.
Get Airfoil by Rogue Amoeba and the free companion app Airfoil Speakers Touch from the App Store.
Open Airfoil Speakers Touch to make your iOS device into a remote speaker for Airfoil. Start Airfoil on your Mac and select System Audio as the content to stream, and the destination NameOfYouriPad.
Switch back to Remote on your iOS device to control iTunes. As long as Airfoil Speakers Touch is running in the background, all audio from your computer is streamed to your iOS device.
If you want to stream video, get Air Video.