The main menu
When your put your R4 cart into your DS and turn the handheld on, this is what should load up. The bottom screen will show three icons: Game, Multimedia, and Boot Slot-2.
"Game" (left icon) is where you go to open all .nds files. "Multimedia" (middle icon) takes you to MoonShell, where you open multimedia files. "Boot Slot-2" (right icon) is what you'd use if you had a GBA expansion pack or other slot-2 device inserted into your DS.