That would be nice, but it doesn't work that way. They can't apply a SNU to an existing booking. You need to call ALL central reservations and make the booking by phone and make a new booking, using the SNU from the start. With the former Fairmont system, you could do it that way, by finding a rate you're happy with, make a booking in advance if you prefer, and at anytime before the stay, apply the upgrade to it. The issue with the Accor SNU is that it's not really a straight upgrade, because (with some limited exceptions) the rates offered use the full Daily Rate, and you generally can't apply the upgrade to any discounted or promotional rate available (even if the rate is publicly available to anybody and everybody booking straight off the hotel website).

There is no way to view rates online. You just have to call ALL, and ask to check rates using your SNUs. The SNUs must be earned and available in your account at the time of booking, and they must have an expiry date after the date of your stay. Once you book, the SNUs will be depleted from your available SNUs in your account management page. If you cancel the booking, then the SNUs will be returned to your account, to be used again against a new booking.