You need to have the Estonia Migrator Tool. In the Tool, enable "Virtual FSX", this allows you to install into P3D just about any FSX add on.

You leave "Virtual FSX" enabled in the Migrator tool, you can close the tool, and Virtual FSX will be enabled until you start the tool and disable it.

As long as Virtual FSX is enabled, you will get the "Legacy Mode" option in your Windows Start menu. Just double click on it and away you go.