I get this error when my project .net framework version does not match the framework version of the DLL I am linking to. In my case, I was getting:
"The type or namespace name 'UserVoice' could not be found (are you missing a using directive or an assembly reference?).
UserVoice was .Net 4.0, and my project properties were set to ".Net 4.0 Client Profile". Changing to .Net 4.0 on the project cleared the error. I hope this helps someone.