How do you get the XDPN card on Pokemon HeartGold?

You can't get the XDPN card in Pokemon HeartGold. The XDPN card is a promotional card from Pokemon Diamond and Pearl. It was given out at events and is not obtainable in the game itself.

There are many other valuable cards you can obtain in Pokemon HeartGold, though, like the "Welcome to Johto" card which you get from Elm at the start of the game.