Tried to open an InDesign file.
InDesign opened but message appeared saying I needed to updgrade plug-ins.
Tried to install this patch: "Adobe InDesign CS5.5 7.5.3 update for Macintosh - All Languages"
But got this message: “AdobePatchInstaller.app” is damaged and can’t be opened.
Related: I recently upgraded my Mac OS to El Capitan.
CS5.5 is unsupported under El Capitan. Best advice is to roll back to your earlier configuration.
This of course assumes you made a full backup, first.