If the computer has been wiped by the thief, Find my [Mac] may not work for it, and it will likely continue to show offline. There is nothing that can be done at that point in tracking the computer.
As pointed out by @Gerry (thanks Gerry), if you wipe your Mac out using Find my [Mac], it may still show up, unlike iOS devices. This is because a Find My [Mac] token is written to NVRAM (persistant memory on Intel Macs), and unless they go in and disable the feature themselves in OSX System Preferences - something they would probably do - you will continue to be able to track the device.
Email notifications do work very well, and it is entirely possible the thief is just waiting for awhile before booting up the computer. Hopefully, he will start it up, and connect to the internet, and allow you to get enough information together to get the police involved.
Most of the time, though, the more tech savvy thieves will wipe the computer immediately, and disable Find my [Mac] which makes it impossible to track the computer.