i understand there were two versions of the Eureka powerdrive mechanism. on the common/later version, all you do is unscrew the cover over the axle/transmission and it should be obvious at that point.
on my Eureka, i noticed the gears were gringing slightly, so i opened it up. you want the gears to engage firmly, yet the clutch able to maintain freedom when in neutral. it sounds hard but will come together when you take the cover off. if i remember, the forward and reverse plates/gears each have their own set screw