Updates:
Status: WontFix
Comment #1 on issue 250 by pekka.klarck: Click button is not working for
slide menus
http://code.google.com/p/robotframework-seleniumlibrary/issues/detail?id=250
Your button isn't technically a button but a span element styled to look like a button. You could try using Click Element to click it. Alternatively you could try interacting with it using Selenium IDE to see how it handles it.
