GameYob emulates the Game Boy/Game Boy Color, not the Game Boy Advance.
If you don't have a DSTWO and want to try and emulate GBA games, check this out:
http://gbatemp.net/threads/gbaemu4ds-gba-emulator-for-ds-i-dsi-entry.281715/
Does that emulator only work in dsi mode? I found a gba emulator once for normal carts but it could only play files under 2 MB