I don’t use Macs but the instructions here: http://audacityteam.org/download/plugins say: “To install new plug-ins, place them in the Plug-Ins folder inside the Audacity installation folder. On Windows computers, this is usually under “Program Files”. On Mac OS X, it is usually under “Applications”. Restart Audacity, then the Plug-ins will appear underneath the divider in the “Effect”, “Generate” or “Analyze” menus.”
Bill just beat me to it, but the FishFillets plug-ins are very old and the VST versions of them are also for PPC Macs only. So they won’t work on OS X 10.6 (which is an Intel system). The way to check (I think) is by right-clicking > Get Info over the Fish Fillets.component file.
I think your only chance is to install Rosetta then find an audio host (VST or AU) that is a native PPC application rather than a Universal Binary like Audacity. You could try running Audacity under Rosetta, but I don’t think it will work.
If you update to OS X 10.7 then there is no support for PPC programs whatsoever.
I thought I’d try running Audacity 2 on my Intel Mac using Rosetta, just to see if the Fishfillets plugins would work, but I have no “Open With Rosetta” checkbox when I Get Info. I know it must be a Universal Binary, because the same download (I think it’s the same) runs on the old PPC Mac Mini.
If you drill down through /Applications/Audacity/Audacity.app/Contents/MacOS/Audacity and right-click > Get Info, that should show you “Universal”.
Universal applications can be configured not to show the “Rosetta” checkbox. If Audacity does not show this checkbox on an Intel Mac running OS X 10.6, I assume we may have done this in order to give better performance (the PPC emulation can make Audacity very slow).
On my Mac (OS X 10.7.3) if I open /Applications/Audacity/Audacity.app/Contents/MacOS/Audacity and right-click > Get Info, I see “Universal” for Audacity 2.0.0. But not if I open /Applications/Audacity/Audacity.app .