You need a "T1" size LED. This is 3mm in diameter at the end that lights up.
Pick any color you like. Get clear or diffused lens depending on your taste, or try both.
Get a data sheet or find the SPEC that gives the operating volt drop of the P/N junction (all LEDs are just positive material mated with negitive material, like in a diode, hence P/N junction). This voltage helps you calculate the current limiting resistor value.
I could give you a resistor value to start with with that info via e-mail.
Or, get a 9VDC battery and a cheap 10K Resistor Pot (25K for blue LEDs) and you can play until you get the brightness you like. Start at 10K and slowly turn the resistance down until the brightness is right for you.
Once your happy measure the pot to see what value worked at 9VDC. E-mail me this info and I will give you the vaule resistor to use in the MB-100 which has a much higher voltage than 9VDC.
Got that