To get svn:ignore, remember that svn:ignore is a property of a file system entry. That means you need to use svn propedit svn:ignore
(to edit the property).
What should you ignore? Anything that's built from something else. All "lock files" that editors might put in place. All temporary files. Some configuration files that require differences between developer (however, you should check in a template with a slightly modified name, like config.template). Basically, anything that you don't want checked in.
It sounds like you are using a non-command line client. Maybe the client is configured to not allow you to modify this file (or it's properties). Maybe it's a different issue. I would recommend installing a command line client, as the errors will be more verbose (and not subject to interpretation like your "grey" color is).
Odds are good you will have to svn:delete the file first (make a copy of it or you'll lose your only copy), and commit the "deletion" back to the repository. Then propedit the directory where the .classpath file was to add ".classpath" to the svn:ignore list.
--keep-local
keeps the files for you, but removes them for anyone else that runssvn update
afterwards – Shrum