[Jared Russell]

I see several .nbm files at the link you provided (five of them in fact).

To automatically install from within NetBeans, however, you want to go to the Settings tab - NOT the Updates tab. Add the given URL as a new provider, then go to Available Plugins to download the SDK. Once you already have it installed, the Updates tab will allow you to download any updates to the FRC stuff, if they are available.

If the above directions still don't work for you, make sure that NetBeans knows about any proxies/etc. that are keeping it from seeing the WPIlib site.