From the product page for the enabler:
"Do you need the AirPort Extreme 802.11n Enabler?
Before purchasing the Enabler software, please check to make sure that your Mac does not already have the 802.11n enabler already installed.
- Open Network Utility (found in the Applications > Utilities folder)
- Under the Info Tab, choose Network Interface (en1)
- In the section 'Model: Wireless Network Adapter," if it says (802.11a/b/g/n), you already have the 802.11n enabler installed. If it says (802.11a/b/g), you do not have the 802.11n enabler installed."