Yup, follow the link already posted to Student Financial Services, then click on "Forms and Publications" at the top and look for "Outside Award Reply Form" -- you can print out the .pdf.
New students probably were asked to turn in the outside award form in May or something, but a lot of students get additional scholarships at graduation and later, so MIT knows that some of the forms will be dribbling in over the summer (and even into the fall). MIT expects you to fill out an initial form and submit a revised form if you get new outside funding; they will tweak your financial aid award accordingly - It won't decrease your overall award, just where the money comes from. I believe that outside scholarships first go to decrease your self-help (i.e. work-study and loan).
If you still have questions, get in touch with Financial Aid
MIT - Student Financial Services (Scroll down to the Financial Aid contacts and find the person assigned to your section of the alphabet).