R4 DS card help!! It wont let me use it!!?

My friend gave me her DS Light and along came a R4 card. I left my charger in Texas and it was the only way I could hook my DS to the computer. Now I'm using my friends charger and his plugs only into the wall. When I put my R4 card into my DS, all it says SD/TF card error. WHAT DO I DO????? Plus I'm only 13 so please don't use big words!