Use GoodPlayer ($2.99) app
tap "Wi-Fi" in the upper left corner
on your PC, open any web browser and enter the address displayed in GoodPlayer's window on the iPad (usually http://192.168.1....)
press submit and wait for the upload to finish
*NOTE* the iPad and your PC have to be connected to the same network, otherwise use the other FTP address displayed below in GoodPlayer's interface.
Well, you do NOT want to use GoodPlayer for 1080p MKV / AVI playback. Its software decoders suck at 1080p. There are much better players for this particular configuration; for example, AVPlayerHD.
Well, I wouldn't consider AVPlayer HD to be "much" better, since it has it's own share of problems decoding 1080p MKVs, video speed and audio volume and sync to name a few.
For getting the files off the computer, you can use Filebrowser. One of my favorite little programs!
Use a converter to convert the video first, and then transfer it to iPad with itunes.
Unfortunately, it only supports native Apple video formats (mov / m4v / mp4) - not AVI's or MKV's.
BTW, speaking of accessing videos on the local network for streaming (or download prior to playing if you prefer), AVPlayerHD, BUZZ Player and It's Playing have decent support for them too. (Albeit at that GoodPlayer is the best. Too bad it has a weak 1080p H.264 / ASP decoder.)
... where "converter" should be a remuxer app (iFlicks, Subler etc.), not a complete reencoder.
I want to say that is what the native format for the native viewer/streamer. However you can hold down on the file and a contextual menu will open up which lets you pick what app you want to open the file in. I will test it out tonight and let you know. I have done AVI's and they will transfer over and play in another player. Hope this makes sense.
Yup, it can pass on files to external viewers - but not over 200 Mbytes. (Another very annoying and silly restriction of iOS.)