Start > Control Panel > Display Settings > Settings > Change colour quality from 16bit to 24bit or 32bit. Its a known issue when the display is in 16bit.
Is your contact list wide enough to display the icon? Have you restarted your computer? Have you uninstalled StuffPlug-NG to see if it fixes it? If you show the preferences window and move it to another location does the icon appear on the contact list?