In the top left, where it states Greetings, Last visited, Private messages. Right underneath there on the left is your current mood, if you click on it a drop down menu should let you pick one and all you do is click on it.
If you can't get it to drop down, if you have one, try a different browser. On my old version of firefox it wouldn't work, but it would in IE, now I've updated firefox (also something you can try) and it works again.
Or, if you decide you want a new browser- I'm always a fan of firefox:
It's Time to Get Personal