The plugin does not have a valid header.

I've tried to download and install with both Mac files. But upon activation, I get this error..

"The plugin does not have a valid header."

Also, when I try to remove the plugin, I can't remove the directory... I get this error...

"550 Can't remove directory: Directory not empty
: /public_html/(websitename)/wp-content/plugins/ckeditor"

But it is empty.

I don't get it?