Welcome to Linux Questions!-
Sorry I'm not the best with the wireless stuff.
I could be wrong but it your just running Ubuntu on a usb stick and it's not installed I'm not sure if the wireless will work. It could be that you need to actually install the Ubuntu OS and than install the driver for your WiFi card and the firmware too.
Is your Mac a Macbook pro 8?
Knowing the exact model and wifi card (nic) you have will help us.
Run this command to find out what wifi card you have:
lspci | grep -i network
I'd wait for a more experienced member to assist you with the wireless.-