When you receive the PIN enclosed in a envelope, login in your Adsense Control Panel and put the PIN at My Account.
Yes, they will post it, and then you type in the code given to you and press submit That's it, but you need phone confirmation as well, so you go the phone confirmation page in your adsense and once you press the button a automated phone call will come (you can specify the amount of time before they call you) and then you type those details as well.
Where do you live? I took 2 months for me to get mine. They said 2-4 weeks but it took 8. That was in 2006 maybe it is much faster now. You have 6 months to enter the PIN so no worries. Just wait for it.