This fee must be paid online before you go to the interview.
It can be paid online at http://www.ice.gov/sevis/i901/index.htm by filling the I-901 online form
After you complete the payment process, an electronic receipt will be displayed. PRINT the electronic receipt immediately! You can use this printout as your Sevis payment confirmation if the paper confirmation sent by them does not arrive before the interview date. Have a printer ready when you pay the fees because once you close the window you will not be able to print it.
The paper confirmation will arrive by postal mail within two weeks.
Instructions/Information at https://www.fmjfee.com/i901fee/students/student/help.htm