How to turn on <tab> usage?

I'm unsure of your intention. Tab and Shift+Tab work to navigate through the controls in the Edit Event window. It doesn't work in the Description free-form text box, though, presumably because those keystrokes are already used for navigation by Thunderbird or the operating system. I know of no way to get around that. You can paste tab characters into the box. Depending on what results you're looking for, you might look at changing the Style from the drop-down Styles list to Preformat, which will give you a monospace font and you could use spaces instead of tabs. You can paste tab characters from the clipboard into the box, which means you could create your columned, tabbed text in a text editor and paste it in. Or, you can paste a table in from a spreadsheet.