mpirun help!

Trying to get a program running across 3 different machines: all macpros running latest leopard.

here's what i enter at the command line:

mpirun -np 20 --machinefile ~/runs/mpiruns/msh_mc citcom ./ridge-input

all the options are correct, but most of the time when I start to run it I get a double password prompt (presumably because I want to start the process on two computers remotely?) and I get asked for my password multiple times even after entering it. Sometimes the program starts to run, other times I get disconnected for entering the password "incorrectly" too many times (even though I am definitely entering it correctly).

any ideas?