On Mac OS, you must press the Enter key (as opposed to the Return key) to paste to a selected cell. The Enter key is commonly found on the numeric keypad of a keyboard but may be located elsewhere, especially on PowerBook and iBook keyboards. Pressing Return merely moves the cell pointer down one cell.
The Paste Options button appears when you use the Paste command. Clicking this button displays a menu of options you can use immediately after pasting one or more cells.