WakeOnLan should be already there. Thing is, it doesn't wake the PC automatically only by starting Xbmcremote. That would be too dangerous to accidentally wake the PC by tapping on the button. However, in the case your PC is off, just click on the "Connect" button, select the host you want to wake up and hit "connect" again. That should wake up your PC (given you either use Zeroconf or have manually added the MAC address when configuring the host)

Another way to use the WakeOnLan feature is to get a NFC tag (Not the smallest one with only 48 bytes), write it with Xbmcremote and every time you tap it with your phone it will automatically launch xbmcremote, wake up the PC and connect.