Assuming you downloaded the nUI+ 5.07.05 Development version you should be able to install it like any other addon.
It's easy to confuse the development version from one that every day users would use. Think of it as like a beta test of the addon and you'll be set. Scott's betas though are pretty good. If you're a pally, warlock or druid you probably will need to find temporary addons to utilise the new power bars but apart from that nUI works with most other addons you may already use barring of course those that nUI provides itself such as action bars and unit frames.
Although, my best suggestion for a new user is to not install any other addon with nUI until you have played with it a bit as nUI provides quite a lot of functionality that you may have currently been using with many addons before.
Once you've played about with it and made your list of missing features you had before, find the addons you used for that and load them one at a time and make sure nUI doesn't have problems with them. Yes, unfortunately that happens, but Scott is good to deal with those type of problems unless there is a reason for the problem in the first place