Colourlight is right, but wrong on that you can use only one car type.  Don't feel bad:  I had the same problem time ago.  When you configure the MIN, at the right tab, where is asking for the car to use, select "None", and then any car enable to carry passengers will load or unload.  I think that instead of saying "none" it should say "any", but it may be very hard to do as Auran never corrected this, even if thousands of us made noise about it.