How do you get the EXPN card on Pokemon soul silver?

You can't get an EXPN card in Pokémon SoulSilver. This is because the EXPN card is specific to Pokémon Diamond and Pearl, and its purpose is to allow you to access the Pal Park feature.

Pal Park lets you transfer Pokémon from the Generation IV games (Diamond, Pearl, and Platinum) into your Generation II game (SoulSilver in your case).

Since SoulSilver doesn't have Pal Park, there is no need for an EXPN card.