this is the terminal i have and the adapter came with it cause its can be used as a negative or positive terminal. the adapter is right next to the terminal on the picture
http://mobile.jlaudio.com/graphics/42___connection
it sounds like your trying to use a positive terminal on a negative post. my terminals came with an adapter so they can be used on either a positive or negative post but some dont so you might need to buy a negative terminal
if you want it to hit harder than your fiends 12" type r's then you need to get something different, like maybe what the other people are suggesting because just cause you get 15s doesnt mean their gonna hit harder than the 12s