I'm not a member (anymore) myself, but from what I understand & remember you can
not buy the tickets through that service.
Sometimes though they put aside a small number (500) of tickets which the members can buy (through the regular sales points) a day or so before the actual, regular sales start. But that's only for home games as far as I know.
As far as I know, the "guaranteed" tickets refer to the new security rules: There are a number of games for which you need the fan card to be able to get tickets. However, the Juve premium membership is a package deal: a Juve membership & a regular, general fan card in one.
So if there are special security measures for the Milan game (which is certainly possible), then you'll still be able to get tickets thanks to your card.
Quite often though, the special security measures don't apply to foreigners, so it's far from certain that you would have needed it.
As for purchasing the tickets: Regularly check the Juve site, they usually put up a message when they're available & how you can get them. Alternatively, keep an eye on the Milan site or
http://www.legaseriea.it/. This last site seems to provide ticket info at times as well. (New since this season, I think.)