You don't have to modify anything. There should be a cable out the power amp section that plugs into one of the speakers. Plug that cable into the IN on the attentuator and a short speaker cable (not instrument cable) from the OUT and into your speaker. Done.
The Rockcrusher is cool, but it's pricey. The Bluesbreaker is only 30w, so I'd actually recommend this guy since it can take up to 50w. I have the 8ohm version that I've used on a Hot Rod Deluxe, Excelsior and Deluxe Reverb (brand loyalty, eh?) and it's amazing. Totally changed the way I used my amps, since I'd never had the opportunity to really drive them hard. Doesn't negatively affect the sound quality at all, and it's like 10% the cost of a Rock Crusher as well as small enough to be easily concealed.