Have you tried entering you current POS in the INS before loading the flight plan? I normally load the flight plan while the INS is aligning.

Another thing to remember, is to drop the last digit of the coordinates you see on the top left of the screen if you press SHIFT+Z.

i.e. I enter these coordinates:

S37^o 40.59'     E144^o 50.91'

as 8(S)37405 INSERT    6(E)144509 INSERT