Don't fret if you don't immediately get your free mystery champion/skin, because it's going to take Riot some time in order to give the gifts to all of the players that qualified for it. Remember the time when Riot gave us all a mystery skin? Yeah, that took quite some time for all of us to receive our skins. (I had to wait 1 week before the skin was shipped to my account)
TL;DR: Be patient for the free champion/skin, it can take some time to ship to your account(s).