In the META-INF/manifest.mf file add the directory location (relative to the jar itself) of your config.properties file to the "class-path" attribute. E.g. if your file is going to be in the same directory as the jar, then add "." to the end. If it's going to sit in ./config, then put this in the path.
NOTE: Remember, manifest.mf files have a maximum line length. Go over it and things get confusing. For more information on JARs and manifests see the Sun guide.
Thanks to Rob Newsome for his help in figuring this out.