Yes, I'm opening a file with a .key extension. First char is U :-(
I open in a text editor. It is bad?
Multibit: Tools / Export Private keys
Did you encrypt it? File has to be unencrypted. I'm 100% certain that's not the private key you're looking for.
If done correctly, there should be a specific formatting that displays the private keys associated with the addresses.
Don't share with anyone the file or it's content.