big button is the one closer to the key. As the bottons are closer to the top, it's the left button.. Sounds like you need a new battery.... did I mention that I also carry an extra battery with me too?
When I got locked out due to weak battery, I could also set the alarm off with the small button, but I couldn't unlock it...
Lotus survival pack: Extra key fob, Extra fob battery, cell phone, AAA
-P