Input Configuration: How do I configure the inputs of the DVMHD (Model 8400/8500) as Contact Closure or Voltage Inputs?

This is done on the front-panel of the DVM-8400 or DVM-8500.  Just go to IO Setup-->Input Type.  As a side note, there is no need to worry about connecting voltage inputs when the product is configured for contact closure operation.  This will not cause damage (as long as this voltage is 24VDC or less).  The only effect of having the incorrect input mode configured is that the input may not work reliably.