FileVault prevents your Mac from booting without the user entering an administrator's password, which means that if a thief were to take your computer and restart it, Hidden would not be able to communicate with our servers to report its location.

Instead, we suggest password protecting your user account, and setting up an easily accessible Guest user account through which the thief can access the internet.

You can read up on how to add and manage additional user accounts on your Mac here: http://support.apple.com/kb/PH14411