rf 7.5, wont let download a/c from knifeedge

I suspect that this thread belongs in the RF7 forum instead of the Realflight Basic forum. That said, I suspect that you're trying to "run" the aircraft file in windows instead of importing it into your copy of realflight.
While running Realflight, click on Simulation...Import...Realflight Archive and select the file you downloaded. Windows doesn't know how to handle an RFX file - which probably explains your question.
 
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