I am using the Toolset Starter Theme, I use awesome fonts to show icon for the menu instead of text, I want to add the ALT text for each menu item, so that we visitor mouse hover the menu, it show the text.
It is the title attribute you need to add (not alt) for the tooltip to show on hover.
The Toolset Starter theme will automatically add these if they are set.
You may be wondering how to set them.
They are hidden by default in WordPress. If you go to Appearance > Menus to edit your menu, then expose the Screen Options tab at the very top of the page where you can opt to display certain advanced fields, including the Title Attribute, which you can then add individually to each menu item.
An alternative would be to design your own tooltips and use Javascript to suppress the native tooltips by removing the title attribute, but, in any case, it is not "standard" and will require you to come up with a custom solution.