This is one of the many known issues of NSM, and despite the fact this issue has been around ever since they released 2011.1 a couple of months ago, it still isn't fixed and they still don't tell you about it or warn you about it when you install. They let you run into the problem and wait until you open a JTAC case. Great stuff, right?
There are a few workarounds that you can try. What usually happens when you see this is the NSM GUI client wasn't able to download the complete schema from the DevSvr.
Firt try this: On Windows in your user folder you will find a folder called ".nsm". Delete that complete folder, just trash it. Then open the NSM GUI client again, making sure you open it with Administrator rights (right-click on NSM and so "open ad admin" or something like that). If you are lucky, the client will try to re-download the schema from NSM server, and most of the time I've seen this issue this helps.
If not, you need to manually copy the schema form the Server to the Client. There is a KB article on how to do this. I don't have it here right now, if you don't find it, open a JTAC case.
Hope this helps.