object:externalId/lookupField is not very clear is it. There's a comment hidden away in the Pentaho code:
// We use an external key
// the structure should be like this :
// object:externalId/lookupField
// where
// object is the type of the object
// externalId is the name of the field in the object to resolve the value
// lookupField is the name of the field in the current object to update (is the "__r" version)
Lets say you're populating a Salesforce Object Foo__c, which has a Lookup field to Contact called Contact__c. The 'relationship name' for that lookup field would then be Contact__r.
On Contact lets say you have added an External ID called Legacy_Id__c and thats what you want to use when populating Foo__c.
What Pentaho would want in the Module Field column would then be:
Contact:Legacy_Id__c\Contact__r
The bit to the left of the slash is telling Pentaho which object/external id to map to. To the right of the slash, its telling Pentaho which lookup/relationship on Foo__c to fill in.