@mgracious I don't remember I was asked for an appeal number. I just logged-in using my FIN (Non-SG citizens), submitted required documents together with request letter from immigration (Im applying for Australia ).
You should click the "Appeal by Non-Singapore Citizens to Apply for Certificate of Clearance". Once you have submitted documents and paid for the appeal, and it was approved, you will reference number (thru email and sms). You will use that reference number to "Apply for Certificate of Clearance" using again your FIN.