Enter /usr/local/bin/pngquant in the Terminal and hit Return, to see if pngquant is found.
If it isn’t found, then find out where it has been installed.
If it is found:
Have you set KM’s ENV_PATH variable to include the /usr/local/bin path? (KM Editor > Preferences > Variables)
If ENV_PATH is not correctly set, then (create and) set the variable in KM’s preferences. For example to:
/usr/local/bin:/usr/local/sbin:/usr/bin:/bin:/usr/sbin:/sbin
Note: You can also choose to set different paths. The important thing is that /usr/local/bin is included, and preferably at the beginning. Otherwise KM does not know where to find the Homebrew-installed tools.
See also Wiki.
Then test with this macro, to see if pngquant is found now:
_[test] Shell Test.kmmacros (1.4 KB)