It says on http://audacityteam.org/download/plugins :
To install new LADSPA or Nyquist plug-ins, place them in the Plug-Ins folder inside the Audacity installation folder.[…] On Mac OS X, it is usually under “Applications”.
If you prefer, you can put them in ~/Library/Audio/Plug-Ins/LADSPA or /Library/Audio/Plug-Ins/LADSPA .
Then Audacity > Quit Audacity and restart Audacity.
Gale