haha. did you look up tutorials? it's okay if you didn't. right click on the little icon in the bottom right of the page. there should be 5 rows of boxes. the first row (three boxes) generates another shimeji to fall from the top of the screen. the second row (five boxes) makes them follow your mouse until they reach it, then they sit down. the next row (6 boxes) makes it so all but one of that particular shimeji disappears. then there's a row that says "IE", and I don't know what that does. the very last row (four boxes) exits that shimeji. makes them all go away.
also, if you want, right-click one of your shimejis. it has some japanese text, and basically it gets rid of just that one shimeji.