I encountered a similar problem using a newer drive on an older Mirage tandem. It looks as if the "drum shaft" was too wide to fit between the notches in the "cam lock knob". I sidestepped the problem by using one of the older drives. Take a close look at the reason yours doesn't fit and see if the shaft is a little too wide.
If it's the spine being too long, try dropping the drive in front first or back first. Hobie has been getting some pretty tight fits on some of the newer hulls. Your dealer may have to adjust the well. Let us know what happens.