Go to Applications>Utilities>Network Utility then click info. Then chose your network interface (en1). This should tell you if you have "N" enabled.
10.5 made the enabler obsoleteI have the same model. Bought it in April 2007. I think it was late enough that I didn't need the 802.11n enabler, but I am almost 100% sure that your MBP at least supports 802.11n. Follow the instructions above to check if it is already activated.
Can't you just look at your box??