Well it's simple. All machines must have mental ray.
So to keep things simple, you have to copy-paste the .bat file on each machine.
Then, on each machine, start the command prompt and run the .bat file. The -j options that is in the .bat file will cause all machine to render different frames.
The tricky part is to make sure that mental ray will have access to the database over the network.
The first thing is to map the network drive where the database is.
Next, if you have SI3D on the machines, you just have to do Get > Scene, and browse to the database, and when asked if you want to add the database say YES, so the path will be established.
But if you don't have the software (but just mental ray), you will have to add it manually. Go in Start > Programs > Softimage Products > User Tools. Select DatabaseDir and click Edit File. Then the informations this way:
DATABASE nameofthedatabase
PATH pathtothedatabase
It could look like this:
DATABASE mydatabase_db
PATH Q:ProjectsCurrent_projectsmydatabase_db
Click Save and Close.
Hope this helps
Salutations - Cheers
Bernard Lebel