Couple of minor clarifications which may help further . .
1. When CCTM says:
The AU plugin components should be in either of the two Library:Audio:Plugins:Components folders (usually the main boot disk library).
Since you are new to Mac, you may not know that there are two AU component folders. One is inside the Library/Audio/Plugins/Components. This houses plugins that will be available to all users on your Mac. The other folder, ~/Library/Audio/Plugins/Components where "~" refers to your home directory (inside the "users" folder) houses plugins that are available only to you, your user account. Since you may find yourself messing within these folders quite often, they are ideal candidates to be added to your left column in the finder window as a short cut. If you are the only user on the machine, it shouldn't matter which folder you put them in, though some installers prefer one over the other for whatever reason.
2. the reason Fish Fillets is not available is that it was never ported over to the Intel Mac platform. When considering older AU plugins, one has to always check to make sure if they are Intel or "UB" (Universal Binary" which will work on intel - UB works on both Intel and the older PowerPC platform. I would suggest that Spitfish from Fish fillets is the most loved and mourned plugin that never made the transition .. .