Controlling Dc Motor With Pwm
This setup will almost work. The pin outputs on the MSP430 are not capable of driving a motor directly, this is due to the current limits on the output pins. You will need to place "something" between the pin and the motor. There are many things that can fill the "something", but in all cases they will basically act as a switch. A simple mosfet is probably the easiest to go with, you just need to make sure you get one big enough to power the motor.

As far how a PWM change the speed across the motor, what you will be doing is adjusting the duty cycle of the PWM. This is essentially adjusting the On/Off time of the motor. The PWM will be running so fast that you wont actually be able to notice the Ons and Offs unless you look at it on a scope, but the effective voltage across the motor will change which results in a change of speed.