Step 4: Choose the Settings
AC is for aluminum - choose "AC".
Set the "Cleaning/Penetrating" setting to the more "Penetrating" side - around 7 on a scale from 1 to 10.
Set the "Air on" to about 5 seconds, if you have the option to do so.
This is the amount of time the gas stays on after the arc stops, to keep your weld from oxidizing/rusting.
Set the "Max Amps" pretty high - perhaps around 250.
For specific settings, check out Miller's settings calculator.
For the curious:
Cleaning/Penetrating is a modification to the positive/negative ratio of the AC that changes the depth of your weld (cleaning is more shallow).
DCEP means "DC, Electrode Postive". This setting is used for balling the tungsten tip, or stick welding.
DCEN means "DC, Electrode Negative", and is for welding steel.
The difference between the two DC settings is the direction the electricity flows - to the metal from the electrode, or to the electrode from the metal. This makes a big difference in the amount of heat the metal absorbs, and the width and depth of the weld.