You open SmileBASIC, you tap Edit Program (or whatever the button says), then you tap a slot button and put in BASIC code. Then you go into direct and say Run. That's how it works. Nothing quite like really old programming languages :D
Well BASIC is a programming language that is really old (it stands for Beginner's All-purpose Standard Instruction Code) from I think like the 50's or 60's? It was just a while ago. As the name implies, it's simpler than say C or Java.
Its a beginners language. Its good for if you want to learn the basics of programming. It is a dialect of BASIC that takes advantage of the 3DS's specs. BASIC is a computer language from the 80s. This is a good game.