Well radial array should be in real time once you click to activate the tool. So you'll have the mesh you want to duplicate active, select the Radial Array tool, and click in the viewport to make it active. Then as you increase/decrease the number of duplicates you should see it update in the viewport. I'm guessing you…
Radial Array (Duplicate tab). Easiest way would be if you selected the loop around which you want to duplicate items, and align the workplane to that selection (workplane dropdown menu).