The meaning of can't and cannot is the same, but there is a difference between the two. Can't can only be used in an informal way (you cannot use contractions in formal texts), whereas cannot can be used formally and informally.
When you click the emoticon box open, you see they all correspond to some signs on your keyboard. Either you memorize what you need, or you cut & paste. Make sure to insert a space before and after each. It should work...