It would help of you told us which format video you are trying to play. Flip4Mac adds functionality for Windows Media only (WMA, WMV).
If you are installing this on older computers are you sure you are installing the right version? Most sites are best semi-hide versions for PPC computers and just have things up front for Intel Macs.
I use VLC and NicePlayer to play pretty much everything.
VLC media player - http://www.videolan.org/vlc/ - general media player that plays just about anything.
Niceplayer - http://code.google.com/p/niceplayer/
Frankly I don't use Flip4Mac. I use the others I recommended. I'm not sure if Flip4Mac works through a web browser if that's how you're trying to play these videos. It's supposed to add functionality to Quicktime Player, but that may just be the application, not the plugin.
I have never worked with DRM protected WMV and am not sure if Flip4Mac will play that.
Getting audio but no video suggests the container (WMV) is working but you are missing a codec for whatever video is used. Again, are you playing these through a browser or actual WMV files on your computer? I always download the files so I am no expert on getting browsers to play video.
Might want to install Perian codecs too.
Perian codecs for Quicktime - http://www.perian.org - plugins to add functionality to Quicktime for playing additional audio and video formats. 2012- No longer being developed but may still work.