How to write en and em dashes?

You need to learn the Unicode numbers of these two characters. They are not actually hard to remember, because they look like years. ;)

• En dash: U+2013
• Em dash: U+2014

To write a Unicode symbol, perform the following actions:

• Press Ctrl+Shift+U (the display will show something like u̲)
• Type the code (e.g. 2014)
• Press Enter

There you go: – —

For a complete reference to Unicode characters, run gucharmap.

When I look up "em dash" on the gnome 3 overview a result comes up that puts it on the clipboard. I think you need to have gnome-characters installed for it to work.