There might be a few branches of HN+ maintained at any given time, simply checkout the source code and switch to whichever branch you want to use. Open up your extensions page in Chrome, check the Developer mode button.
You'll see a bunch of options open up, select "Load unpacked extension" and browse to the folder you downloaded the source to.
HN+ is pretty rough around the edges, and the code itself isn't very "neat" at all. But it does what I intended it to do, make Hacker News look a little different.