I know how to download plug ins for my Mac G4 Powerbook using OSX
10.3, but once I get them what do I do with them  to make them work
within the system?

Which application are you downloading plugins for? plugins generally
go in one of the "library" folders, but it varies.

I am attempting to get Stuffit to open documents that are stuffed, but
it gives me a message that says, "Sorry, Expander requires Stuffit
Engine 8.0.0 or later. Please reinstall from the original installer."
One adviser told me to go to the library and find CFMS support and
from that install Stufit Engine Shell, but where do I install it. I am
not sure this is the cure. According to a lookup on my hard drive, I
have Stuffit  8.0.2, but Stuffit Engine Shell, indicates it is 7.0.4  
    Please help.   Thanks

If all you have to do is uncompress documents, I would reccomend doing
a search for 'stuffit' and dragging the engine, and related program
components to the trash. Afterwards, go to
http://www.stuffit.com/mac/expander/index.html and download a free
trial, which comes wth a fully functional version of Expander. This
will have version 8 if the Stuffit engine shell.

good luck, hope I helped.

In OS X, Internet Plug-Ins are usually installed in the Internet
Plug-Ins folder of the global Library folder (/Library/Internet\
Plug-Ins). The common plug-ins are already installed there, and the
plug-ins you'll be downloading will almost always come with installers
that will place the plug-ins there for you. As a previous respondent
said, you may need to download <a
href=http://www.stuffit.com/mac/index.html>Stuffit</a> in the event
that it is not already installed.