I have the same unit and can confirm that if you go in via Media Centre PC you can select MyGica and then ATV520E and it finds it just fine.
BUT.
and for me it's a big but.
The stock remote doesn't have a Play, pause, stop etc etc button available so you have to tap OK, then use the menu which is a PITA, especially when you have a Harmony remote with those buttons just sitting there.
I've read elsewhere you can use the Pivos DS as a device which gives the extra buttons but I can't get them to work.
I have a cheap ($7 ebay special) MCE remote I bought to use with a RaspberryPi and I thought I would try and see if it works:
http://www.ebay.com.au/itm/390776490215
The answer is yes, this remote plugs in and just works, not all of the keys do anything but at least the stop and pause buttons work as do the arrows and the enter button. Sadly you need to have the USB IR adaptor.
(also it's worth noting that fast forward and rewind buttons don't seem to work).
What I'm thinking is that I could setup my Harmony remote as the MyGica unit then manually learn the extra command I want from the MCE remote then leave the USB adaptor plugged in and get all the buttons I want working.
So it's a solution, but not the ideal solution. I'd still love to get the ATV520E to directly accept commands rather than having to use the USB adaptor, if anyone has this working I'd love to know how.