First you have to add a new device from Pluto-Home web administration on you Core. the link should be something like : http://<yourcoreIP>
You will be prompted for a user and a password ( those from the installation process of the Core ).
Then go to Media Directors / Add Device and choose "Generic PC as MD".
The the new device will be added in the list of Media Directors.
You have to set the MAC manually for your MD ( the format is 00:11:22:33:44:55), then do a Update and finally, Setup Diskless MD.
That's all, you wait for the installation too complete, do a quick reload router and you can use it.