When you install Microsoft Visual Studio 2008 or 2010, you might need to select the X64 Compilers and Tools features. The 64-bit features might not be selected by default, so you must manually select them. On the Options page of the setup wizard, select the X64 Compilers and Tools checkbox under Visual C++. For ARX compilation you do not need to select the checkbox for Itanium Compilers and Tools, if available.
If you previously installed Visual Studio 2008 or 2010 without the X64 features, you can add them to your current installation by using Add or Remove Programs in the Windows Control Panel. Double-click the Microsoft Visual Studio 2008 or 2010 entry in the Programs list to launch the setup wizard in maintenance mode.