Aha, I've found the problem. I thought I had the latest version installed as I tried to install today, but apparently Unity downloaded 2 versions of the plugin into 2 separate subfolders under Tasharen Entertainment folder in Library. So even though it downloaded the latest version, it then only imported the older version 2.3.3 every time I tried to import since that version.
So if anyone else has trouble updating NGUI, just check your unity asset store folder (Library/Unity on the Mac) and delete the old versions and redownload from the asset store.