There are several ways to do it -maybe the easiest is to click <> in editor and use html entities (moodle converts them to unicode characters) or directly unicode characters (copy - paste)
≥ is ≥
≤ is ≤
Some more examples:
and if you want to test unicode characters for example http://rishida.net/scripts/uniview/ is a nice site...
And if you need a lot of special characters you could install for example Enriques plugin http://moodle.org/mod/forum/discuss.php?d=20864