Obtain Secret Key at Toys 'R' Us in Canada this month
Update: TRU Canada has since taken down the announcement.
The TRU Canada Web site provides only basic information, but it is likely the item will be distributed wirelessly via Mystery Gift.
According to Bulbapedia, "the Secret Key opens up a secret room at the Galactic Eterna Building, containing five appliances necessary for Rotom to change between its six forms."
Thanks to Bulbagarden Forums member Outrage for the tip.