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Make sure home sharing is turned on in iTunes on your mac, and enter the same details on your iPad under settings / video.
Now, when you go into videos on your iPad you should see an option for shared videos - this will show you all the movies on your mac and you can play them on your iPad
There are a few ways. Assuming the DVDs are ripped, you can either play them from iTunes and home share your library like said already or, if the format is not comptible to iTunes you can use Plex, load the media server on your PC and then load the client on your iPad. So long as the PC is on you can watch your movies in almost any format at home, on the road, just about anywhere you have a decent internet connectiong on your iPad.
If your videos are on your mac and organized in a folder, you can stream all of them to your iPad using a server on your mac (free) and a player app on your iPad (2.99$) called AirVideo. It's really awesome ! Brought all of my old dvds back to life !! If the file format is not compatible with the iPad, it converts it on-the-fly so you can still view it seamlessly. Best purchase I made in a long time !