You have PokeSav right? I don't know any alternative to uploading wondercards onto a save file other than PokeSav so I'll just try and help you with that. But keep in mind, last time I checked, PokeSav from HG/SS does NOT allow events/wondercards to be uploaded. DPPT works fine though.
PokeSav Plat -> Events, click edit (Bottom Left) -> Mystery Gift Edit (top right) -> Load PCD (download a WC package), load the PCD file, then go to "Load PGT" (mid left) and load the other file from the package. Then click "Enable mystery gift" if you haven't already.
Overwrite the file, open up the save and try. It should work.