/Library/Audio/Plugins/Components if they're audio units or
/Library/Audio/Plugins/VST for vst
they have to be compatible with the platform (eg., Intel or PPC) or they'll be silently ignored, and of course, Windows compiled VST plugins don't work either.
Logic doesn't support VSTs anymore, either, only Audio Units.
It does seem a little weird that Logic's manual doesn't tell you where to install third-party stuff, but I guess that's why most plugins you download (and certainly the ones you buy) tell you where they need to be installed.