POS should be connected to the positive side of the battery
and should be turned on/off by the ignition system. The regulator will use this for battery
voltage sensing, so if you can figure out a way to get it controlled by but not powering the entire ignition circuit that will give you better regulation.
NEG to wherever your negative battery connection is made.
EXC goes to the field connection on the alternator
[EDIT] the reason to turn the POS on/off with the ignition is because whenever the POS has voltage the field current
will flow. If your alternator is not spinning it will just run down your batteries. You can also do this by hand with a little toggle switch if getting into the ignition circuit is a pain, but you have to remember to turn it off when you shut down the engine. [/EDIT]