I guess you mean that the drive carrier (the mount for the optical drives) won't seat into position?
If you got the OWC drive, then you need to follow the video that shows you how everything fits, including how to route the additional cables (assuming that you are changing one of the PATA drives to an SATA drive. If you are keeping both as PATA, then there shouldn't be an issue with "too many cables"

Anyway, should be simple to see what is preventing the seating of the optical drive carrier.
Can you tape extra length out of the way of the carrier? (Duct tape or package tape should be helpful there!)
Another question: Do you have the latch lever up (unlocked), on the back panel, when you try to seat the optical carrier?
I think if you can't seat the carrier, then you also can't move the latch all the way down (?)