Very simply put, in the userbar at the top of most pages on EcchiDreams:
Click on the smiley face icon:
Which will load this:
Click on the icon you want, and it will add the text under that icon to the "Currently Feeling" box below, which you can then edit if you wish (up to 48 characters). Then click on update, and that's it.
To remove the mood, just open up the moods panel and click "Remove"; that's it.