Hi Suds,
In addition to registering the FTM software you also need to activate the 6 months "free" subscription.
If you look under the Help option on the menu bar you'll see that there is a "Activate Ancestry Subscription..." option. Just select that and follow the instructions.
If I remember correctly you'll need to enter your credit card details at the time but won't be charged anything. Just remember to cancel your subscription before the 6 months are up otherwise they'll automatically renew the subscription for you and debit your card at the time.
Ian.