NO, you do not need to buy it. It comes with the Extreme, and should have been installed when you installed the Airport software. If you went through the install, you should have 'n' on the MacBook (assuming you installed it on the MacBook).
Open Applications->Utilities->Network Utility, go to the Info tab, choose "Network interface (en1)", and look for 802.11a/b/g/n at the bottom.