Yes, you need to enable the static checking. Go into the project properties and visit the "Code Contracts" tab. Of course you'll need to have installed the static checker extension first, which IIRC is only available for Visual Studio Ultimate. See the user documentation for details of the exact options available.
Note that this happens after the initial compilation phase, so sometimes you'll see a delay between the first part of the build and the results of the static checker. (It's not really a "compiler" error - it's a "contract checking" error.)