If you want to send a letter via postal mail, the address is:
Marvel Entertainment, Inc.
417 5th. Avenue
New York, NY 10016
I am not sure whom to address the letter to; perhaps editor-in-chief of the X-Men titles? If someone has a current comic and can check who the editors are, please post here (that would be a huge help!) Also, if you have another suggestion as to whom to address it to, please post here. Otherwise, you can send it to the address exactly as it is written above, but I'd really like for it to get into the proper hands.