Another Data Grid problem

[Paul Foraker]

I have a data grid with 11 columns. I want to display an image in each cell
of column 1. I've go the key data in the cells. When I click the + button
to add a custom behavior, nothing happens.


Steps:
Choose pointer tool.
Click on data grid.
In Property Inspector, select Columns.
Click to select column 1 (already by default).
Click the + button in the lower right of the Property Inspector window.
Expected Result: the custom property editor window should open
Actual Result: the + button highlights on the click, but does nothing

Did I miss a step?

Is there an alternate method of adding a custom behavior?

LiveCode 6.1.2 | Build 2016
Mac OS 10.7.5