Are you sure the RX is bound to your TX and working correctly? Have you used this RX in something else previously or is it new?
You could try hooking something straight to the RX to make sure the RX is working correctly. Just hook an esc/motor to the throttle channel or a servo to one of the other channels and make sure that the TX and RX are talking to each other correctly.
If the RX and TX are both confirmed communicating with each other I'd check your MultiWii config to make sure you don't have it setup up to use PPMsum if you're not using a summing RX - or that it is setup to use ppm sum if you are using a summing RX.
when you connect your board to the computer it wont show RX activity until you plug in the lipo (but you probably knew that).
Then, if you get that going, make sure that your throttle is below 1100 otherwise it will not arm. I had this problem with my TX so I just put arming on a switch (so i didnt have to use the rudder) The rudder trims may need to be changed too, but idk what the values are for that.