Edit your .bashrc to add the desired directory on the PATH environmental variable.
export PATH=/usr/local/google_app_engine/bin:$PATH
then, either start new terminal or do,
source ~/.bashrc
Now try to run the script from anywhere.
Another way you can do it without even touching the .bashrc would be to create a symlink by doing something like,
sudo ln -s /usr/local/google_app_engine/bin/script.py /usr/bin/script.py