First off steve......The source should be connected for you to legitimately be trying to measure anything....even if that is just a 240 ohm resistor.

Note however that the part you specified is an N-Channel part and the far more normal way to connect this type of part is to connect the source to GND when the GATE drive signal is referenced to GND in the microcontroller.

You are also going to have difficulty saturating the the part with a 3.3V drive signal from the microcontroller. This part is spec'ed at a RDS(on) of 0.042 ohms when the VGS is @ 4.5 volts. The data sheet shows that at a VGS of 3.5V typically you are going to have an ON resistance of well over 2.5 ohms.

Michael Karas