Open the Lowender disk image (.dmg) that you downloaded from the reFuse website. You will see two versions of the Lowender under the heading "Install Lowender Plug-In":
Note that, despite the ".vst" extension on the Pluggo 3.6 version, both versions of Lowender are compatible with RTAS, VST, and AudioUnit hosts.
If you had previously installed the Lowender demo, open the Pluggo folder (you can double-click on the Pluggo folder alias as a shortcut) and trash the demo version of Lowender before launching your audio host application.
If all went well, Lowender is now installed and ready to use. For your reference, that Pluggo folder alias should have put the Lowender file in /Library/Audio/Plug-Ins/VST/Pluggo/. Note that this is the Library folder that you see from your root hard drive folder, not the Library folder within your User folder.
If you'll be using Lowender as an AudioUnit plug-in, you must run the "AudioUnit Scan" application, installed by Pluggo into your Applications folder. This will add Lowender to the list of available AudioUnit plug-ins.