Band-in-a-Box is about as simple a program as you can get--it is basically a programmable karaoke--you can open up a song file and just click and play, or choose musical style, tempo, and key from pull-down menus--you can also type in the chords to anything you like and do the same pull down arrangement, and enter the melody from whatever MIDI device you happen to have connected--it is easy to save the whole thing as a standard MIDI file by just pulling down the "File" menu and choosing "Save as a Standard MIDI file"--

What it doesn't do very well, and, most likely, what you are having trouble with, is let you enter the other parts--it really isn't mean't to do much more than let you choose from pre-programmed parts(you can enter parts, but it is basically a step entry system, like the old, old drum machines)--

Given the limitations, it is great to play along with, and even to create listenable demos of songs that you have written(as long as you are happy with the default arrangements)--I used to create "music minus one" type tapes for my guitar students to practice their leads and scales--But if you want to enter other parts, you will need a sequencer program of some type--