I did send an email to them.
Are you applying this year? Maybe someone who has applied in the previous years can confirm if there is not so much as a field and support preference "form", as individual entries (on page 1 and 9) to be filled up in the online application, as sydfloyd suggested.
But then i do not understand why they expressly mention on the website (http://web.mit.edu/physics/prospective/graduate/index.html
) that a complete application includes both the online application and
field and support preference form, if it is anyway part of the online process.