Which Nexus 7? If you have the older 2012 tablet, you need to download the BLE enabler app, or use a custom ROM that enables BLE. The 2013 Nexus 7 already supports BLE.
Incidentally, I'm writing this because I'm using CyanogenMod on my Nexus 7 2012 tablet, and a recent update actually enables BLE even though it was supposed to have been enabled. Now everything works perfectly.
