There are two ways: coding and testing.
ZotPad code is available on GitHub. You can fork the code on and start developing on your own computer. Once you have something worth testing, do a pull request to merge the changes back to the main repository. If you want to really commit more time on ZotPad, it is possible to join the project. The possible profits from selling ZotPad on the App Store will be divided among all the developers. If you have more questions about any of this, email email@example.com.
A less involved way to contribute is to join as a beta tester. See http://www.zotpad.com/beta for details.