Douglas,
Start by switching to "Daily" view.
Highlight a task row – the Unassigned button become
available.
Select any cell of a specific day on any resource row - the Assigned
button become available.
I agree that it's not intuitive. We are planning to change
that in one of the upcoming HF's and add Drag and Drop (task to resource)
capability in V6 (Apr 2013).
Regards,
Uri