I am unable to modify objects in the Design tab.
1. At the upper right corner of the Labeler window, look for the word Properties written sideways in a box. If that is there, mouse over it to reveal the Properties pane. Once shown, you can click the pushpin icon at the right side of the Properties title bar to keep it open.
2. If that sideways box is not there, click the Design tab, then in the toolbar, click the Reset Docking button. This should bring back the Properties pane.