I had exactly the same situation. I missed Platinum by about 5 or 6 nights. I knew I was going to gold. I called Platinum Cong and spoke with them. They offered a challange of 18 nights in 3 months. I accepted. 2 days later I recieved a email saying sorry with the terms in it. I was not gold (still at Plat) and the terms clearly stated it was for Gold to challenge to be Platinum. So I waited until that day when my website changed and GOLD was my new status. I sent a email requesting the challange with a copy of the terms.
I recieved a reply stated "since I had been offered and was made aware of the challenge it would be offered to me." I had 3 choices of a 3 month spread to choose from, sent in my selection and recieved a confirmation email back.
I am gold now but will have my 18 nights before the end of this month and look forward to the benefits again.

Send the email.