If you're running a windows version later then 98, it shouldn't need drivers as it acts as a mass storage device.
Before starting the DS up, make sure the DSstyle Cartridge is in the first slot, the GBAstyle cartridge in the second one and that the USB cable is connected between the GBA and the PC.
Now start the DS and click the option to start the DS game (N-Card).
Now choose the bottom option USB Mode.
Windows will tell you a mass storage device was found and probably ask you how to open it, choose that you want to show the files/folders on it. Or just open My Computer and choose the new drive thingy over there.
There you can just drag&drop stuff like you normally would.