How do I restore a lost icon from my toolbar

I somehow lost the highlighter icon from my tool bar. How do I restore it?

Try Tools - Customize. There you can modify tool bars and menus, and keyboard shortcuts to your liking.

I tried that but when I click modify I get another window asking for a file name. What file name?

You get a file Dialog when you click on Tools > Customize ?
You should be getting this Windows