the executable makes things extremely extremely easy. download the latest release for either linux or windows, and unzip it somewhere and run (on linux you'll need to allow execution first, as per usual). alternatively you can build from source, which might be the only way to run on mac?
it'll take a bit to launch the first time, because it's generating 12 images as the original breeding stock. subsequent times will be faster. all images generated will be in a newly created folder called "data", which you can safely delete when you want a clean slate.
ganbreeder will launch in your default browser, at http://localhost:8742/, and you just use it normalstyle. on windows, it'll also open a shell window (closing it will end the process). on linux, you can end the process from the system monitor instead.
the tradeoff for all this ease of use is that the inner workings get a bit more obscured... of course you can, and should imo, browse the source files, but any changes are also behind building the thing again (not too bad, but still).