This happened to me. If it will not run while offline but it will run while you are connected try this. This is what he told me to do when it wouldn't work for me.
Open this file :pixelator\resources\app\js\bundle.js with a text editor, and remove line 162, which looks like this:
checkForUpdates(false);
This will remove any internet usage by Pixelator.
Hope this helps!