This command installs (or updates) the plug-in that Eudora 6 uses to communicate with SpamSieve. It also disables any other Eudora junk mail plug-ins, as they would conflict with SpamSieve. The SpamSieve Eudora plug-in is installed in the folder:
/Users/<username>/Library/Application Support/Eudora/PlugIns/
For more information, see the Setting Up Eudora section of the manual.