Streaming video entails using AirPlay. The minimum requirements for AirPlay are: 1) iTunes "server," 2) AirPlay "speaker," and 3) Wired or wireless network.
Sorry, but I'm still not 100% sure what you are trying to do. Is your goal to stream HD video files, located on an AirPort Disk (HDD attached to an AirPort Extreme), to the iPad? ... or from the iPad to some other device, like an HDTV?
If it's the former, I am aware of at least three different iOS apps for this: 1) Air Playit, 2) Air Video, & 3) StreamToMe.
Another option is: VLC Streamer
Finally, you may find the following Wired article, about using the FileBrowser app, useful as well.