The Safari applications needs to be terminated for Flash Player to be installed. Closing the browser window does not terminate the application. To terminate the application, mouse-over the Safari icon in the Doc, press CTRL and left-click at the same time. In the menu that displays, select Quit. Safari is now terminated and installation can proceed.
I do not have a CTRL tab, are these letters from my key pad?
CTRL = Control, the control key. Alternatively, you can use Activity Monitor to terminate Safari: