From the Insert tabclick on the drop-down arrow beneath the Symbol icon in the Symbol group.
Click the Symbol button and popular/recently used symbols will be displayed. Select the appropriate character by clicking on it.
If the desired symbol does not appear, click More Symbols... and the Symbol box will be displayed
Change the Font to provide a different range of characters e.g. choose Symbol, for Greek characters. Once the intended symbol has been found, select and and press the Insert button.