You can not directly access CPAN modules through ppm; ppm can only install precompiled ppm modules, which are not generally available through CPAN. If you have a compiler compatible with the one used to compile your Perl installation, you can use the CPAN module to install CPAN packages.

The ActivePerl HTML docs for PPM lists several ppm repositories. There are quite a few about. One I've found useful for modules that Active doesn't carry is at http://www.bribes.org/perl/ppmdir.html.