At first, you need to switch to x64 in Visual Studio. In the toolbar of Visual Studio, there are two dropdown fields - one to switch between Debug and Release, the other to select the architecture ARM or x64, wich must be set to x64.

Now you need to addd a new AppPlatform connnection for the x64 Platform. This can be done by either adding it inside the Connection Mannager (Tools - innovaphone SDK - Connection Manager), or by selecting the "<New AppPlatform connection...>" from the dropdown list inside the New Remote Connection dialog.

For this new AppPlatform connection, you need to give the IP address of your x64 AppPaltform as well as the associated PBX. After adding the new AppPlatform connection, you should be able to select it when creating a new RemoteConnnection.

Hope that helps.