What custom shape are you trying to create?

A pedal line with v lift on the end of it.

I have the same issue with the Windows version.

Is this the shape you want?

What process are you using to choose the shape as your custom smart-shape?

Yes this is the shape but after I select it in the custom dialogue box - it reverts to a default straight line

I usually use Finale on my Windows machine, but I opened the version on my Mac (El Capitan 10.11.6) and had no problem choosing SmartShapes, Option-clicking on the Custom icon (the star-in-the-hexagon) scrolling to the line I wanted, clicking on it to highlight it and then clicking Select. 

With the Custom icon still selected, I double-click and drag to position the SmartShape. It works for me, so unless Sierra is causing a problem, I do not understand why your version does not work as it should. Presumably you have already tried restarting you computer (which would mean restarting Finale), a frequently suggested catch-all solution.

You do have the option to contact MM directly via the "Submit a request" link above or via their phone help line (during business hours) under the Finale Software pull-down on their Contact Us page at http://www.makemusic.com/contact-us/?_ga=1.263438542.376967974.1459532219 

Yes I have re-booted with no difference. Why would Sierra create an issue for me? - I've sent the question on to MM today and will hope for a reply sometime next week. Thanks Jim