with Plexus you need to make sure that you have a Transform effector for the group. The checkboxes in the "Create Anchor" window will only appear enabled if there is a Transform available for the group.
I'm in the middle of an OS upgrade so if this doesn't work give me a couple days to re-install all my plugins and check how Elementary works with the latest Plexus.
Thanks a million Kevin.
You are a real gent!
Scott