Is the gauge not marked + and -? Have you checked for 12V at the pink wire?
The sender is a variable resistor to ground. You'll need a meter to check it. As it warms there should be less resistance.
To test the gauge: Remove the wire from the sender. Connect a jumper wire to the sender side of the gauge. Turn on the ignition. The gauge will do nothing. Ground the jumper wire. The gauge will go to full sweep.
If all that works and it still doesn't work hooked up make sure the wire from the gauge to the sender isn't open.
