I recently ordered the 2010 World Edition, and when I installed the program, it gave me the option to activate the Ancestry membership.
If yours didn't, then when Family Tree Maker is running, look at the menu along the top of the page, click on help. One of the drop down options should be "Activate Ancestry Subscription".
It is a pain having to input credit card details, I can't see a way around that, but I've got a large note next to my monitor saying when the membership is due to expire. Just remmeber to cancel a couple of days in advance!