The latest released version is available for download from the Zen Cart site, via this link. If you need basic installation help, and neither this information nor the included readme does not help you, please visit the plugin's Zen Cart support thread.
Ultimate URLs (a.k.a. USU) allows store owners to use alternate URLs. The URLs generated will include "human friendly" names in the user's browser's requested language and contain shortened versions of certain parameters.
v3.1.0 and later supports stores based on Zen Cart versions 1.5.7a and later and requires no core-file overwrites.
Note: If you are updating USU from a version prior to v3.0.0, there are, unfortunately, some hand-edits required to remove those previous versions' changes to Zen Cart core files. See here for additional information.