Two things:
1) You are using the overall Library. This should work, but you are the only user of FCP in this machine, I'd suggest you place those files in your own Library (~/Library/.... instead of /Library/...)
You can reach it in Finder by holding Option and choosing Go->Library
or use Command-Shift-G and type ~/Library
2) Camera LUTs and Custom LUTs are not the same thing.
I think that what you have are Custom LUTs, but I'm not sure.
These are added using the Custom LUT effect
3) It could well be that some of these files are not properly formatted.
Where did you get them from? Are they paid, or free? A cube file is a text file with a specific content. You may open it in TextEdit and have a look. Don't change anything.
What do you see in the first lines?
There could be some comments (lines starting with #)
A title,
TITLE ...
A line indicating the size
and a (possibly very long) set of lines with numbers - that is the actual lookup table