Here are a few things to try.
If you are trying to load a MPG4 video, make sure it is encoded using H.264. This encoding is required in order for MPG4 video to playback in SlideShowPro. Also, H.264 support in the Flash Player was added in "Flash Player 9 Update 3" (aka v9.0.115.0) in December 2007. Check to see what version of the Flash Player you have installed here. If lower than 9.0.115.0, install the latest version of the Flash Player through Adobe.com.
There are number of FLV encoding applications out there, and not all of them publish FLVs with correct (and sometimes empty) metadata. Metadata is needed so that SlideShowPro knows what size your video is, the duration, etc. If you load a FLV video and the size seems incorrect, or behaves erratically, we recommend using the standalone "Flash Video Encoder" application that comes with Macromedia Flash 8, Adobe Flash CS3 and CS4. You're guaranteed to get correct metadata every time using the official video encoder.
You can learn more about Adobe Flash Video Encoder here.
Another possible issue is server MIME type. Most servers know what to do with a .flv file, but not all (like Windows Server 2003). One way to test this is to load the FLV file from your server in a web browser. If the browser window fills up with text, MIME type is the reason. Contact your web host or server administrator and ask that they please add (or check to make sure) that the FLV file type is recognized.
