newbie question on snow/Rmpi

I have a network of four machines set up.  I'm having trouble spawning
my slaves on these machines.

All the examples I have found so far use makeCluster with type="MPI",
and I guess I'm missing some kind of cluster configuration in my
environment variables because all my clusters are formed on the
machine where my initial session is running.

Can anyone refer me to a link that illustrates how to set up the
environment for MPI (listing node names, number of slots, etc) that
will allow the makeCluster command take advantage of the other nodes?

Thanks,
Whit


simple example:

library(Rmpi)
library(snow)

cl <- makeCluster(2, type = "MPI")