Hi Larry,
I had the same problem which I have finally solved.
In Airnav, start a recording
In plane plotter, options/directories/radarbox log file make sure this points to the directory that you are saving the recording into.
I use VISTA which has it's own way of creating files. If your using the same look for:
AppData\Local\VirtualStore\Program Files\AirNav Systems\AirNav RadarBox 2009\recording it should be in your usual profile directory.
Hope this helps
Dave