I don’t think PackMan works in MODX 3.
(You could create the package on a local MODX 2.x installation though, and use that package in MODX 3 as well.)
A snippet in MODX is more than just the code. There is also some meta data like name, description or what category it belongs to. All that information is then written to different files in a format that MODX understands together with information about the package and zipped.