The easiest way to set up NUnit tests reporting in TeamCity is to add NUnit as one of the steps to your Creating and Editing Build Configurations and specify there all the required parameters.
* TeamCity-provided tasks may have different syntax/behavior. Some workarounds may be required to run the script without TeamCity.
In addition to the common test reporting features, TeamCity relieves a headache of running your NUnit tests under x86 process on the x64 machine by introducing an explicit specification of the platform and run-time environment versions. You can define whether to use .NET Framework 1.1, 2.0 or 4.0 started under MSIL, x64 or x86 platform.