And here is the longer thread that discusses the offer (answer posted courtesy of dejamo)
The Marriott Rewards number is only Monday through Friday, so you'll have a few days wait.
If you were asking how to add nights - then the tried and true - Marriott Credit Card, if you haven't secured one yet, you receive elite night bonuses for getting the card and if you already have one, you receive one night credit for each $3,000 spent. Don't forget to register for your birthday elite night credit (if you're that close) and depending on your shortfall, you can find some rooms at excellent weekend rates around the Thanksgiving and Christmas holidays. And last, again depending on your shortfall and circumstance, you can always call and ask for an extension in hopes of getting a generous service rep (this of course, should be the last effort, but it sometimes works and it certainly doesn't hurt to try if you are definitely going to fall short of the 50 night target).
If you're asking how to secure points to exercise the elite buyback offer - you can buy points for $12.50 per 1000 points, so theoretically you could buy a Gold status (if you're already gold) for $312.50.