First, there is no such product as Music Maker Pro, sorry! There is Music Maker Plus and Music Maker Premium; I assume you have one of these two?
The serial key for Sound Forge Audio Studio should have been included in the confirmation email you received when you made the purchase. Have you checked the whole email; quite often the serial key for additional products might appear towards the end?
If you still cannot find it you will ned to email email@example.com, with details of your original order and ask that they send it to you.