I am no expert, but this is my understanding of WoL.
When a device is asleep it is not visible on the network by your router, in that it will not have an entry in the ARP table.
That is if you send something to the device by name or IP address your router will not know where to send it.
So, to wake a device you have to broadcast a message (magic packet) throughout the local network, targeted at the MAC address of the network interface of the device.
This will only work inside your local network, i.e. it does not work across the internet without using complicated network tricks. (You cannot broadcast magic packets throughout the internet and have your router pick it up and rebroadcast it to all the devices on your local network).
If you use JRemote and enter your MC Access Key this will give JRemote your HTPCs MAC address to be able to wake it.