It sounds like your gnome-panel is locked down. You can remedy this from gconf-editor. Start it from the quick launch dialogue (ALT+F2) or from the terminal: Once the editor has opened, navigate to "apps" > "panel" > "global", and uncheck the key called "locked_down".
Alternately, issue the following command in a terminal. (It does the same thing):
Code:
gconftool -t bool -s /apps/panel/global/locked_down false