X5s are equipped with newer version of immobilizer system, so in order for remote starter to work you will have to hide spare key somewhere near receiver or find immobilizer by-pass which is additional to the remote starter. I haven't seen immobilizer by-pass for 2007+ BMWs.
In addition, most aftermarket shops will not install remote starters on newer BMWs, Mercedes, Audi’s and other high-end European cars due to possible complications and conflicts with electrical system.