Where are you looking? They would be on the Menu Bar in the Write window. Open a write window. If the menu bar is hidden pressing F10 or the alt key will make it appear. Then select View-Toolbars and turn the missing toolbar on.

The toolbar is not missing. It gives me the following options: File, Edit, View, Options, Tools, Help.

When I right click to open the 'customize' box, there is no "insert" icon to use.

That is the menu bar when you are creating a Plain Text message. You cannot Insert or Format in a plain text message and is the reason is is called Plain Text.

The option to always do HTML is found under Tools-Account Settings-Composition and Addressing if you want to change it for all messages.

Holding the Shift key when clicking Write will toggle from plain text to HTML or the other way if set to HTML already.

FYI: You cannot customize the menu bar in any screen.

I really appreciate your help! Issue solved!