Under Lion, I had added Dropbox and the Activity Monitor to the Login Items section of the Users and Groups control panel. After upgrading to Mountain Lion, these were removed, and I cannot add them back via the + button, via dragging and dropping or via the dock-->options-->Open at Login.
The problem is on my Macbook Air 11inch late 2010. It IS NOT a problem on my mid 2011 Mac Mini running Mountain Lion.
Anyone having similar problems or thoughts on how to correct this?