.GBA onto .NDS?

How do I get a .GBA file to work on my R4DS Card rom?I just downloaded Space Channel 5: Ulala's Cosmic Attack (GBA) and it won't work on my R4DS card? I thought GBA games worked on DS?Sorry if this question is very unintelligent :D

GBA games cannot be played on a slot 1 flashcart such as R4. You need a slot 2 flashcart (goes in the GBA slot) to play GBA games on ds.

How annoying!Thanks anyway :D