why is ppm: command not found?
"ppm" is the name of the package manager used by ActiveState's ActivePerl.
The Ubuntu ppm package is the "Power Policy Manager," a completely unrelated thing.
Unless you are using ActivePerl on your Ubuntu install, you do not want to use their PPM utility. If the package you're trying to install isn't available in Ubuntu's repos, or you don't want to use it if it is available, then using CPAN really is your best bet.