Not dumb at all. That is a bug on the Mac but can be worked around in the following manner. If you enter in a value and find menu items greyed, click on the centre of one of the tool settings dials ( left panel ) to bring up the text entry box then press escape to dismiss it. This will return the menus to normal ( as will opening and closing any other similar dialog ). It has to do with dialogs, so the reason it's stopping being greyed out at various points for you will be related to the opening of a pane of some sort most likely. I've noted this down in the bug list to be addressed in a future update.