It's due to video acceleration. The computer just puts a window on the desktop and let's the video drivers play the video in that window, after the desktop has been rendered. The video is being sent directly to the monitor and can't be screen-capped. You can turn off video acceleration in Windows Media Player by going to Tools > Options > Performace and sliding the slider at the bottom all the way to the left. You can't maximise it but you should be able to capture it.
If you turn off WMP, after you capture the frame into Imageready, the image should just be black. If so, then this is the problem. (The fact that the video plays in Imageready says that all you captured was the "Play Video Here" window).
Another option would be to get Media Player Classic - it's free and has a Save Frame As... function as well as a Frame Advance Button.
http://sourceforge.net/project/showf...ckage_id=84358