To add x64 and remove the default AnyCPU configuration, follow these steps:

1. Open the configuration Manager

2. Select New... from the platform combo

3. Choose x64 for the new platform and click OK.

At this point you are good to go. You will notice that your project has been switched to x64.

However, to avoid any confusion you can remove the AnyCPU platform from you configuration with the next three steps.

4. Select Edit... from the platform combo.

5. Select AnyCPU and click Remove.

6. And confirm.