On Windows systems, you can choose to install the RTAS or VST versions. The RTAS version is installed into Program Files\Common Files\Digidesign\DAE\Plug-Ins. The VST version is, by default, installed into Program Files\Steinberg\VstPlugIns. You will however have the opportunity to change this directory during installation.
On Mac systems, you can choose to install the Audio Unit, RTAS, or VST versions. The Audio Unit version is installed into /Library/Audio/Plug-Ins/Components, the RTAS version is installed into /Library/Application Support/Digidesign/Plug-Ins, and the VST version is installed into /Library/Audio/Plug-Ins/VST/reFuse.
Now launch your host application, and check that Lowender is showing up in the list of available plug-ins. Lowender will request your authorization info when you insert it in a track.
If you are running Lowender as a demo, just select "Run Demo Mode" to bypass the authorization window. Running in this mode, your plug-in settings will not be saved when you close a session, and the Lowender's audio will drop out periodically.