If you actually went with Pro (bitbucket access), then yup, drag & drop the NGUI folder is the way to go. Otherwise double-clicking on the package will import it.
NGUI is an editor extension, so a lot of the code relies on the existence of UnityEditor.dll for proper functionality, not to mention the fact that NGUI has a lot of platform-specific #ifdef's that won't be visible from within your DLL.
I do not advise using external DLLs with NGUI (or at all, for that matter).