TO SUMMARIZE
You must be able to log into the GTS and trade normally for this to work.
If you are sure that you can do the above, send a Pokemon by dragging and dropping a .pkm file over the sendpkm.py file. It will then tell you to change your DS's DNS to what it tells you to.
To do that you can start up your Pokemon game normally, and at the load save screen, scroll down to the Nintendo WFC Settings, and go into that.
Then, select Nintendo Wi-Fi Connection Settings and choose what connect you're using currently. Go into that and scroll down until you see "Auto-obtain DNS". Change that from Yes to No, and go down one and input the DNS you were given to you by the sendpkm.py.
Go back and save and stuff. Now go into your game, and go to the GTS. Go in as normal and wait (it takes awhile to connect) and a Pokeball should fly from the sky and into your game.
Congrats, you've followed instructions!