how do I place an ad in the last minute for rent category - or does a listed ad automatically go there when the time is within the 45 days?
Which are you asking about -- Last Minute Rentals here on the Bulletin Board, or in the Timeshare Marketplace?
The Bulletin Board is not connected to the Timeshare Marketplace. If you want a Last Minute Rental listing on the Bulletin Board you have to post it there yourself. The LMR forums are forums just like this one. If you know how to post here, which you obviously do, you know how to post there. Please review the forum-specific posting restrictions shown at the top of those forums before posting there.
I don't know about the Marketplace, but somebody should be along soon that does.
In the Last Minute Rental Forum, you are just making a regular post. The time is posted in red at the top of the LMR Forum, and you need to check yourself and make sure your date is within that time frame (no more than 45 days out.)