Type "sudo cp 40-open-ephys.rules /etc/udev/rules.d", then type your password.
Now you can both use the GUI and make changes to the source code. We recommend reading through the developer documentation before doing so.
Any changes you commit can be synced with your GitHub repository by typing "git push origin master" from any directory inside your project. If you change something that you think would be useful for others, you can submit a pull request to the Open Ephys version of the GUI.
If the GUI won't start on Linux, it's likely due to problems communicating with the audio hardware. Your computer's sound card is what generates the callbacks that drive data acquisition, so the GUI can't run unless it establishes proper communication with your audio device.
If you're running the software inside of gdb (which is recommended, at least until all the kinks are worked out), you may see the message "Program received signal SIGSEGV ... juce_String.cpp:203" after printing "Got audio device." With certain hardware, you'll get a segfault before this point is reached. Either you have the wrong drivers (or no drivers) installed, or (more commonly) the audio hardware is in use by another application.
How to troubleshoot:
sudo alsa force-reload
in a terminal; this will refresh the sound server, and may fix your problem.