Proximity Trigger Enter Filter

Question

I am trying to detect with a Proximity Trigger, the entry of a specific object.  I initially set the object reference to "anything" but birds continue to trigger it causing premature and uncontrolled actions.  Per the SDK, one can specify "Reference" and then list the object in the "enter filter" property.  When I do this the only choices in the elements box are AI and MobileScenery objects!  I wish to detect a flour bomb (flour_bomb), which is in miscellaneous simobjects folder.   Can this be done?  How?  Any help or advise is appreciated.  Thanks
Jake