Where is the notes app on nexus s? Is there a native notes app? I cant find it
There isn't a native notes app for the Nexus S because it doesn't have TouchWiz UI. I'm quite certain the voice action 'Note to self' actually sends an email to yourself (I don't personally use that action, not sure it likes a British accent)

There are loads of third party apps though. For example Scribb.it is full to the bursting with features, AK Notepad is quite a good simple one or Evernote has a strong following and quite a few more features.