Clifford is correct in saying that this is an advanced feature. Join paths are automatically created for basic patterns, but the patterns for automatic generation are limited, so an editor is required. However, the open source tool will verbalize the join paths without the editor installed. Entering join paths is very similar to entering derivation rules, and is part of the pro toolset.
In this particular case, you would be able to enter a join path and have it verbalize correctly, but it would not yet map to an implementation.
There is more discussion about the 'pro' version in http://www.ormfoundation.org/forums/p/1054/3321.aspx#3321, including a request for feedback on how to start making the work-in-progress available to a broader section of the ORM community. Feel free to pipe in on that thread if you want to.