i have the same tx (7c, 4yf) but a g4 system. guessing the plugs are the same
your 7c has to be in the trainer mode to work (menu, scroll up, and click trainer. have to stay in that mode while useing)
the cord only works one way (the end with the adapter, has to be on the buddy box)
your other transmitter (buddybox) has to be on. cause the cord wont suppport the power function

try that and it should work just fine (works for me)

o btw make sure your buddybox has all the reverses set to normal (not reversed)

go fly