First click on the Cell you want to add the symbol to.

Next you will want to go to The Ribbon and click on the Symbol Icon.

This brings up a Virtual Keyboard full of Symbols and Special Characters.

Simply select the one or several you wish to use and double click to enter them into the selected cell.  So in this example you can see I am getting carried away and trying out various fonts and languages… 

However, this is a great tip to know if you need to get that spreadsheet done by 5pm for Mr. Lumbergh!