You probably don't have all the fields of the jobs filled in correctly.
MEncoder, on which iriverter is based, is sort-of unstable on Windows, so some videos may cause it to crash.
Make sure you have the correct firmware installed and that you are looking for the movie in music mode. See Section 1.1.1 for details.
Try using the default video bitrate of 450 and audio bitrate of 128.
After the video starts playing on your iriver, rewind it about a second or two. This tends to get everything back on track. Otherwise, try lowering the video bitrate to 320. If you still get problems with sync play around with the manual sync function of iriverter as documented in Section 3.3.2.