I strongly recommend that you start with a clean slate when installing nUI... often users will have a collection of old mods, saved variable data and other things that will interact adversely with nUI.
So... to start... follow these simple steps...
Go to the [ World of Warcraft ] folder
Rename [ WTF ] to [ WTF.saved ]
Rename [ Interface ] to [ Interface.saved ]
Log in to WoW but do *not* enter the game
If you see an "AddOns" button at the character select screen, go back to step #1, you are in the wrong [ World of Warcraft ] folder
If you do *not* see an "AddOns" button, enter the world on any toon
Exit WoW again
Unzip your nUI download (the ZIP file) into the [ World of Warcraft > Interface > AddOns ] folder
Log in to WoW again, but do not enter the world
If you do *not* see an "AddOns" button, go back to step #8, you installed nUI incorrectly
If you do see an "AddOns" button, enter the world on any toon, everything should be working correctly in nUI now.
My advice is always to wait before you start installing other mods again... find out what nUI does on it's own before you grab a mod that does it... odds are nUI already does it and the other mod is just a "habbit" of yours.
Two things I do recommend people install pretty much immediately is either DBM or BigWigs and either the combination of Recount, Omen and the nUI_InfoPanel_DualStats plugin or Skada (which does the job of all three) -- I also recommend Clique for those who like to click-cast -- nUI is compatible with all of these mods and, for most players, you probably do not *have* to have any mods other than those. That said, nUI is compatible with most mods, so if you *must* then you should be able to use pretty much any mod with nUI. For those who want the extra help with quests (though that's not so necessary anymore with the 4.0 changes) nUI will work with QuestHelper, Carbonite and LightHeaded.
If you have trouble locating the correct [ World of Warcraft ] folder (step #2 above), I would recommend you right-click the launcher icon on your desktop and then select "Properties" to see what directly WoW is installed in.
For a Windows XP machine, it is normally [ C: > Program Files > World of Warcraft ]
For a Windows Vista or Windows 7 machine, is it normally [ C: > Users > Public > Games > World of Warcraft ]
If you are having problems getting nUI installed correctly (step #9 above), then I would recommend you install either WinZIP or WinRAR and use those tools to unzip the download file (usually a name something like nUI_5.07.08_Plus.zip showing the version number and release type). Once you have WinZIP or WinRAR installed, use that to open the download file, then "extract" all of the contents of the file into the [ World of Warcraft > Interface > AddOns ] folder -- make sure you tell WinZIP or WinRAR to preserve the file folders that are contains in the ZIP file (the default setting).
As an alternative, in Windows Vista and Windows 7 you can just open the ZIP file in the Windows Explorer (though my experience has been this is unreliable) and copy and paste the [ nUI ] folder from the ZIP file to the [ World of Warcraft > Interface > AddOns ] folder.
Either way, if you have correctly installed nUI, when you are done, you should be able to dive down your directory tree to [ World of Warcraft > Interface > AddOns > nUI > Layouts > Default ] for either nUI or nUI+ -- for nUI+ you should also have the folder [ World of Warcraft > Interface > AddOns > nUI > Layouts > Default > UnitPanels > Raid40 ]
What people don't get is that I am, ultimately, an artist at heart. My brush has two colors, 1 and 0, and my canvas is made of silicon.