Sorry to hear you are having problems with the trial. We are sadly not able to reproduce the problem.
When you select the Chroma Key option on the Modify Panel do you correctly see your video background change to a checkerboard? If so can you then play this back successfully in popVideo 3?
If everything is as it should be in preview mode, which export options are you choosing? Are you selecting to output in popVideo format? If so, have you checked your range is set to all and the resolution is correct? Also if you are able, do try a different source video as there could be an issue with the video itself.