From ECFMG :
Eligibility Period Extension
If you are unable to take Step 1/Step 2 CK during the three-month eligibility period assigned to you, you may request a one-time extension. You may extend your eligibility period only through the next, contiguous eligibility period (the next eligibility period that does not overlap with your original eligibility period).
Example: An applicant's original eligibility period is April-May-June 2010. The eligibility period may be extended only through July-August-September 2010.
You may request an extension on-line using IWA. Payment of the appropriate fee must be submitted with the request. If you take the exam during your original eligibility period or if you do not take the exam, you will not receive a refund or credit of the eligibility period extension fee.You may request an extension of your eligibility period only once for each exam registration. You may request an extension of your eligibility period only after your eligibility period has begun. Requests received at ECFMG more than 25 days after the end of the original eligibility period will not be processed. For specific information on deadlines, refer to IWA.
Additionally, you must continue to be eligible to take the exam during the extended eligibility period (you must still be officially enrolled in a medical school located outside the United States and Canada that is listed in IMED and the "Graduation Years" for your medical school must be listed as "Current," or you must be a graduate of such a medical school and your graduation year must be included in the school's IMED listing). See Eligibility for Examination.
If your eligibility period is extended, NBME will issue a new scheduling permit reflecting the extension. You can obtain the new scheduling permit by accessing IWA. You must bring the new scheduling permit to the test center on the date of your exam.
If you have a scheduled appointment during your original eligibility period and need to cancel and reschedule the appointment for your extended eligibility period, you must cancel or reschedule at least five business days before your scheduled appointment to avoid a rescheduling fee. Requesting an extension of your eligibility period does not cancel a scheduled appointment. See Rescheduling.
If you are unable to take the exam during your original or extended eligibility period or you are unable to extend your original eligibility period, you must reapply, including payment of the appropriate fee(s), to take the exam