Are you sure it's not asking for L I L O? That should be in the Plugins/dictionaries/LILO folder. Is it really missing? If it is, reinstall. If not, try trashing your prefs.
I was thinking the same thing as Peter.
A quick google of the ULO.InDesignPlugin doesn't reveal much, but a google of LILO.InDesignPlugin shows a link to an indesignsecrets thread which talks about L I L O...
and the long and short of it relates to people trying to open CS3 files in CS2...
so not quite sure why you would have those problems in CS5.5.
Another possibility is that it got turned off, I suppose. It's not required for ID to run. It should be listed in Extension Manager in the Help menu...
The fix was to restart my iMac, after which I re-opened iD CS5.5, system detected path to iD had changed (it had not knowingly been changed), corrected itself, and now the LILO plugin error has departed. Thanks all for help.