Your screwed just like me you need an adapter for rear gear can't find one on internet that fits a bike kit.
You should send yours back as junk since you can't use both discs. Only front!
Reorder from eBay: disc brakes for MOTORIZE BICYCLE.
The kits come with 2 rotors 2 adapters, 2 calipers 2 cables with housing and 2 brake levers. And price is much less than $78.00 found 1 last night for just $15.00 last 1 should have ordered it its not on there this morning.
Most prices rapidly change from a row of $22.00 then jump to $48.00+
If your frame is not pre suited for disc your gonna half to order bike brackets there cheap around $5.00 but look at shipping detail they may arrive from Hong Kong around January 2 way to ducking long.
Some Mormon from Zootah manage to ship his I did not order for $15.00 just rotor and calipers but hit with shipping came out to be billed $39.99 hit the return button he sez, just keep them I'm f*** $40.00