802.11n should be enabled automatically.
In each Radio Profile (802.11a and 802.11g,) there should be a High Throughput Enable knob. That is the master switch for High Throughput (802.11n) to be enabled.
Under each SSID, there is a HT SSID profile, which determines if 802.11n will be active for each SSID. Again, it is on by default:
Top reason why you will not see 802.11n throughput:
- You are using WEP or TKIP (neither type of encryption support 802.11n) - Only Open and AES support 802.11n