I am running InDesign CS5.5 and trying to open an In Design File but getting an error message. " Cannot open the file "PDavidLCover.indd". Adobe InDesign may not support the file format, a plug-in that supports the file format may be missing, or the file me be open in another application."Please help I really need to get this file open.
Since you've shown us the folder contents it's a good bet it isn't in use (no lock file). And it doesn't look like the file is mis-named as there doesn't seem to be anything else there that would be the .indd file (but just in case, open it in TextEdit and paste the first few lines here so we can see what the file header says), so the most likely case is the file is damaged. Is that on a removable device of some sort?
You can try the tool at Repair corrupt InDesign Adobe files on Mac OS X or send me a link to the file by private message and I'll try the recovery tool I have for Windows.