The Motor service gui is being updated.  It has been designed to support multiple motor controllers. This is nice, since there can be a little complexity in attaching a motor to a motor controller.

For example an Arduino can directly drive a simple 2 bit H-bridge connected to a DC motor.  Or a motor shield can be used, which has its own motor port configuration.  Other shields and bridges might have different configuration.

Adafruit Motor Shield

Arduino driving simple H-bridge