My wonderings about the subject, concerning MP:
http://forum.1cpublishing.eu/showthread.php?t=23483
Offline in FMB the detonator selection is straightforward issue when building the flights.. just select from dropdown menu... The low level and dive detonators work ok.
But MP ... Plane -> select correct plane -> Loadout -> Bombs -> select the correct bomb -> do your settings -> Save on every menu level .. and still it doesn't seem to do anything. No detonation.