A couple of things for reference:
nMotorEncoderTarget and nSyncedMotors currently will not work in Robot Virtual Worlds; this is because they depend on values generated by the NXT brick that are not yet emulated by Robot Virtual Worlds. However, the nMotorEncoder command does work and can be used to move the motors a set distance with precision.
We have a video trainer curriculum
dedicated to the nMotorEncoder/nMotorEncoderTarget functions that you can view as a refresher; it is under the 'Movement -> Improved Movement -> Target Distance' section. We also have a rundown of the NXT Motor Functions
on our wiki that can also be used as a quick reference guide.