How do I get unit tests to run on a x64 platform

I'm assuming that you are using Visual Studios testing framework since you mention no other. In VS2010 you can force the testing host to run in x64. Build your assemblies as Any CPU or x64 then set the test host to x64.

http://msdn.microsoft.com/en-us/library/ee782531.aspx

In VS 2012 and above you can select the default target platform from Test menu under Test Settings|Default Processor Architecture