Go to the Disney World web page.
Go down to the bottom of the page.
On the right hand side, you will see "Contact us."
Click on that and it will take you to a screen where you can send an e-mail.
I had several compliments a few years ago; much pixie dust on my trip.
I received an e-mail the same day that my e-mail had been received. I received an e-mail a few days later saying my e-mail had been forwarded to the resort. About a week and a half later, the manager of the resort called me directly, thanked me for my e-mail and said he had forwarded it onto the HR department to place in both of the CM's files.
I am not sure about what your complaint is, and I am sure it is legitimate, but most likely it would have been best done while you were at the resort/park, etc.