Mackyboy. As Pete said earlier,
You can only add these VC's to FSX native models. With Skyspirit/Project Opensky/TDS - the model will always have an 'x' in the model name.
There is an FSX native model of the TDS 737-700 but I don't think they've released an FSX native 737-800 yet. Most FS9 models will display in FSX
BUT FSX native models are not compatible with FS9 so would not be posted as FS9/FSX.
This article explains how to determine if a model is FSX native.
http://www.flightsimworld.com/forums/topic/165025-how-to-use-default-fsx-vc-with-an-addon-plane/I suggest opening the MDL file in Wordpad as the TDS files won't open in Notepad for me.