It is recommended to install in a fresh 5.1 image.
Older Squeak versions are no longer supported.
This is a 5+ Mb download, with documentation and tests included
1) you need the "muO dependencies" package, providing all mandatory third-party code:
2) unzip the muo-cache.zip archive and place the muo-cache folder in the same directory as your fresh image.
3) proceed to the installation of muO (either from a SqueakMap browser or manually by dropping the muO sar archive in a Squeak image)
Note that muO is an application for Squeak: it may not coexists happily with your other packages; it makes important changes to the look and feel of Squeak.
You should always start from a fresh image.