To try any of the examples, copy them over to:

    $XDG_DATA_DIR/share/nautilus-python/extensions/
or:
    ~/.local/share/nautilus-python/extensions/

FYI, it is still possible to load plugins from the old locations.

Then restart nautilus.

Hint: if you're testing an extension that you're developing, it may
be useful to start a 'private' instance of nautilus, like this:
 $ mkdir /tmp/testing
 $ export TMPDIR=/tmp/testing
 $ nautilus --no-desktop