There is no slot-2 on a DSi - slot-2 is the GBA cartridge compartment. Only the DSPhats and DSLites have it. The only way to currently play GBA games on a DSi is to purchase an iPlayer and install the GBA emulator, which is not quite perfect playback but acceptable most of the time. You can also wait for the new Supercard DSTwo that's coming out at some point, which is supposed to have GBA/SNES emulation built into it, but other than an iPlayer there is no alternative at this time.