1. Install GIT from http://code.google.com/p/git-osx-installer/

2. http://josevalim.blogspot.com/2008/05/footnotes-v30.html – Jose Valim is now maintaining footnotes

3. How to install?

cd myapp/vendor/plugins
git clone git://github.com/drnic/rails-footnotes.git footnotes
rm -rf footnotes/.git

In summary, you are better off installing the plugin from GIT instead of SVN if you are using rails 2.1.x or edge ( at the time this post was written)

UPDATE

If you are receiving 500 Internal Server Error when having errors on your views then please read below..

If you are running on Rails 2.1.x, you should use Footnotes v3.2.2:

    cd myapp
    git clone git://github.com/drnic/rails-footnotes.git vendor/plugins/footnotes
    cd vendor/plugins/footnotes
    git checkout v3.2.2
    rm -rf ./.git

If you are running on Rails 2.0.x or Rails 1.x, you should use Footnotes v3.0:

    cd myapp
    git clone git://github.com/drnic/rails-footnotes.git vendor/plugins/footnotes
    cd vendor/plugins/footnotes
    git checkout v3.0
    rm -rf ./.git

Remember that in Rails 1.x, after filters appear first than before filters in the Filters tab.