Mark,
From recollection you need right click on the file and save it to ...\my documents\sky demon\aircraft. It will be then visible when you next start sky demon. When you select the aircraft tab, it should be there.
As to the mogas issue, this really does not have much impact on weights, but may be a little slower in the air as the 100LL would provide that little bit more power. You can update the data once it is in your aircraft tab, and change to suit you.