[UPDATE 2014-04-27] This is unnecessary for VS2010, since you can use
To change the target platform, manually edit the platform tag in the *.csproj project file. The platform tag can be titanium, anycpu (default), x86 & x64.
in the Configuration Manager to add the x64 or whatever target you want. See How to: Configure Visual C++ Projects to Target 64-Bit Platforms on MSDN.