If you want to install for all users, simply copy the file into the folder:
If you want to install just for your user, simply copy the file into the folder:
You may need to restart the QuickLook manager. To do so, you'll need to open a Terminal and execute:
If you installed it before updating to Mavericks (OS X 10.9), you will need to install it again after updating to mavericks. Download the latest release and install as usual.
Here you have it QuickLookJSON.qlgenerator