You need Prewikka 5.0.0 or higher.
The git repo for Prewikka apps is available on GitHub and can be cloned like this:
git clone https://github.com/Prelude-SIEM-Contrib/prewikka-apps-helloworld.git
Once the source code downloaded:
python setup.py install
Then restart Prewikka.
HelloWorld is now available in "?" menu.
The code is released under the GPLv2 license. See the COPYING file.
Please feel free to propose your own Prewikka apps by submitting a pull request.