Draken 35J 5.1 and 35OE 1.1 are designed for Prepar3D v5 but are compatible with Prepar3D v4 and with Flight Simulator X (SP2 and later). Some Prepar3D specific features will not work in FSX, such as the dynamic landing/taxi lights, and the switch sounds in the cockpit. Cockpit configuration will not be saved between sessions in FSX.
The previous versions 35J 5.0 and 35OE 1.0 were designed for Prepar3D and will not work in FSX. Upgrade to the latest version to get FSX compatibility. Upgrades are free if you have purchased a previous version in the same series.
Draken 4.1 requires Microsoft Flight Simulator X SP2 or later. It can be installed in Prepar3D, but it will not work as intended and some major tweaking is needed. You are on your own there.
Draken 3.2 requires Microsoft Flight Simulator 2004 (FS9).