how do you use Det Packs?

Select them in your inventory (Navigate through using I & O), then use them with U. This places them on the ground. Set them off with B.
Be advised that pressing 'b' detonates all detpacks in the vicinity. So when you are standing at a safe distance, ready to blow the detpac, make sure you are not standing rack that has a detpac on it, for it will blow as well. Happened to me once I only saw the energy cubes (which I didn't need since I was maxed out). I must say that Silencers make great human torches though.