Hi Tom, the problem is that the ArcTangent command will only work on 2 coplanar curves, and your curves here are not on the same plane.

If you switch the view to split view mode and then look in the Right side viewport, you can see the blue curve is kind of swooping down at an angle there, while the red curve is horizontal in that view.

In order to get ArcTangent to work on these you will need to squish them down so they are both on a common plane. You can do that by selecting the 2 curves and then in the Right side viewport, drag the edit frame corner grip and drag it down until you get "flat" snap as shown here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3378.4

Hope that helps explain it! Thanks, - Michael