What version of Windows you are using?
If you are using Windows Vista or 7, disable the UAC.
See instructions here:
http://www.unitronics.com/data/uploads/VisiLogic_Software_New/Disable%20UAC%20Vista%20+%20WIN7.pdf
In U90 Ladder, go to Contoller > M90 OPLC Settings, click on Advanced, set the Parity to Even, the Stop Bits to 1, and the Data Bits to 7.
check the box of Force PLC settings to