do i add transmission fluid through the dip stick on my 2012 dodge durango

Yes, and you will have to use a funnel with a long skinny spout. Add only 8 ounces at a time being careful not to overfill because suctioning it back out is not easy. The dipstick usually states the proper fluid and you check it with the engine at idle speed in neutral. Might be Dexron IV.

When should you add it? I checked it, on level ground, when it was hot and it was to the top cold hole. Should I add some until its to the bottom hot hole when hot? Its on a HEMI 6800 series 5 speed. Thanks,

Yes, check on level ground. At idle check fluid. Put the dipstick back. Hold brake on and go through all the gears slowly, then put it in P and check fluid level again. Add 8oz at a time until you reach about the hot mark.