Ok so what to plug in :

Tailmotor signal >>you just plug it all in the reciever as before, no changes.
Power Check >> you just plug it all in teh reciever as before, no changes.
Main motor signal >> do not plug it in, it will be replaced by the plug from the brushed to brushless converter
Servo's >> all 3 in as you used to.

So what did i do :

The power cables ( red and black) from the mcp original ESC i soldered that to the power wires off the new brushless ESC.
The plug with 3 wires from your new ESC goes to the brushed to brushless converter, the plug from this converter will go in Throttle on the receiver.