As I gather from your comment you did run the exe file and it installed directly in to your fs9.
If .................. you had read the intructions you would have readed that it was adviced to install the exe to a predetermined folder and from there manually install the aircraft in fs9.
INSTALLATION of the BETA D18S AMPHIBIAN
Create a Temporary folder on your Desktop or anywhere.
Run the d18samphibian.exe
When prompted, browse to the temporary folder to install to.
When done, copy the d18sam aircraft folder to your FS9/aircraft folder.
Go get wet!
So that is the reason why the aircraft doesn't appear, it should be listed under Beechcraft MSA in your aircraft selection when installed properly.
so maybe a little lesson in this for you, READ THE READ ME FIRST
As for where the files you have installed went ..................... check your main fs9 folder maybe it's installed there.
fled