Mystery Gifts come in a variety of free pokemon. To get these Nintendo has to first do a promotion with a retailer.
For example during pre release of pokemon black and white, the three legendary cats were offered at gamestop for download from the DS download station there. Also ToysRUs gave away Ash's Pikachu for free.
So unfortunately you can't get a gift until something like that is done again. You might want to check pokemon tournaments, sometimes they offer other mystery gifts to participants and spectators.