Extra marks over letters

You can do it through insert>symbol and then select the letter you want. Often this is a button on the toolbar with an omega symbol on it. You can also use the alt key plus a numeric code keyed in on the numberpad, but you need to know the number. You can get this though from the insert >symbol dialogue box (it will have it somewhere down the bottom).

First, understand that these "extra marks" actually constitute entirely different characters for the computer. Every character has a corresponding numerical code, and the ones not directly on your keyboard can be made in a couple of different ways, as Jess mentioned above. Some time ago, I took five minutes and made a list of the Alt-keypad number codes (above number 128 for the special characters), which I keep with my laptop computer. The Ä is made with Alt+142 on the square numerical keypad located on the right side of standard keyboards. It doesn't work with the linear numerical keys across the top.