Clone tool question

Is there a way to select source for the clone tool?  I'm trying to fix some seams and I've got it set up correctly but the old tried and true way of selecting source doesn't seem to work as expected (coming from Photoshop where you use the alt key to select source).

Also in my early attempts I'm finding that the cloned bits are coming out too dark (from where the little box is gliding over which I guess is the source but it's it's not correct) 

Thanks so much

Richard