The red light is telling you that the motor loading is such that you need to peddle. To avoid damaging your drivebelt etc , it's always best to peddle at first then engage the motor while you are on the move.
The belt slippage can be cured by rotating the motor, it usually sits with the flat spot/heat sensor attached at the lowest point.
Be careful not to overtighten the 10mm bolt that fits through the "cuff", you can damage the surrounding plastic. Try shifting the motor sideways ,or rotating it, to see if you've got the tightness correct.
Don't forget to make sure the gearbox end sits on the "wall" part, most people have a mod bracket or tie-wrap to keep it all in place.