that midibox looks interesting for sure!
as for the buttons, yes only one leg of + and - is required to make it work.
its best to just test continuity of the button when pressed. if you have a digital multi-meter with continuity checker, attach the probes to 2 contact points and see if it goes through. that way you can be sure your wiring it correctly.
on the resistors, if they are always on as you say, then you need to know what voltage is coming in and plan on a forward voltage of 2v for Red and green and 3.5 to 4v for Blue and white. also the output is needed so go for around 20ma for regular leds. you can calculate the resistor needed here