A ClassPath in a jar file manifest must be relative to the jar file itself, not to the local file system. The convention is to simply list the jar file name without a path and plan to keep it in the same directory as the jar file you're creating.
So, you'd want to change your entry to:
Class-Path: tar.jar activation.jar
Another quirk of manifests in jar files is that you must have an extra line feed character at the end of the file.