Hi Joram, 
that's exactly what i"m trying to do: creating new "Activiti" project with Maven in Eclipse. Here's the steps that I follow : 
1 - creation of a maven project 
2 - Selection  of the activiti archetype : 
      - Group id : org.activiti + 
      - Artifact id : activiti-engine
      - version : 5.16.4
3 - Specification of archetypes parameters : 
      - Group id 
      - Artifact Id 
- Package, version , etc..
Then the error below occurs : 
Unable to create project from archetype [org.activiti:activiti-engine:5.16.4 -> ]
The defined artifact is not an archetype