If you have them on your computer's iTunes then you should be able to sync them to the iPad's Videos app by selecting the iPad on the left-hand sidebar of iTunes (if you have iTunes 11 then you can enable the left-hand sidebar via option-command-S on a Mac, control-S on a PC) and then using the Films/Movies tab on the right-hand side to select and sync them.
If they don't sync then you can try doing Advanced > Create iPad Version (File > Create New Version > Create iPad Version on iTunes 11) and try syncing that version.
If you can't get them into your computer's iTunes then you will need to convert them first.
Pad2, the new iPad Supported Video Formats & Movie Formats
- H.264 video up to 1080p, 30 frames per second, High Profile level 4.1 with AAC-LC audio up to 160 Kbps, 48kHz, stereo audio in .m4v, .mp4, and .mov file formats;
- MPEG-4 video up to 2.5 Mbps, 640 by 480 pixels, 30 frames per second, Simple Profile with AAC-LC audio up to 160 Kbps per channel, 48kHz, stereo audio in .m4v, .mp4, and .mov file formats;
- Motion JPEG (M-JPEG) up to 35 Mbps, 1280 by 720 pixels, 30 frames per second, audio in ulaw, PCM stereo audio in .avi file format