Or, one solution I find quite easy is to add 'character palette' to your panel. Right click on some space on the panel, select add to panel, locate 'character palette' on the utilities group, and select 'add'. It will add kind of 'palette', and it has lots of characters to add. I usually add superscript two (²) by clicking on the appropriate character, and paste it into your text (it copy to sort of 'clipboard', I guess).
EDIT: by the way I did ask someone on linuxmint IRC channel, and he/she said something sounds like 'ASCII characters doesn't supported by Linux' or so. Unfortunately, I didn't save the chat log.