I found a forum post about Visual C++, Visual C++ 2008 Express Edition And 64-Bit Targets , but how do I develop x64 applications under Visual Basic 2010 Express Editio
I don't have Visual Basic 2010 Express installed to check, but if you go to the project properties and view the Compile tab, do you have an "Advanced Compile Options" button? Click that and then look at the "Target CPU" drop down. By default it should be "Any CPU", which means that .NET will run as x86 or x64 depending on the OS that the app is running on.
Since .NET code is not native code, the target platform doesn't matter as much as it would with a C++ project. The recommendations I've seen suggest targeting x86 will all your .NET applications unless you have a specific reason that you need the extra memory addressing that you'd get with an x64 application.