Originally you use "F2" to activate the Aimbot Feature but if you go into the 'config.mw3esp' located with the program where your downloaded it and open it with NotePad or any text editor then scroll down to the bottom and you'll find this
You'll see a HTTP address (Website address) located in the description text just under the title 'Hotkey Settings' which will tell you the corresponding key code for the key you what to replace the default with. All you have to do then is replace the default 'KEY_AIMBOT_ACTIVE = Keys.F2' with 'KEY_AIMBOT_ACTIVE = Keys.##'
(## = what your new key is)
Sorry for the long explaination but I hope it helps,