Hi,
Usually, additional aircraft have to be installed by unzipping the downloaded file and placing the unzipped content in X-plane 11\Aircraft.
No need to run the installer.
Once in X-plane 11 aircraft selection menu, you sometimes need to tick the box "show aircraft from previous versions" for them to be displayed.
We can't give more help unless you give more precision about where you downloaded your plane.
You can probably find installation instruction in a readme or doc file inside the downloaded archive, or on X-plane.org forum.
Regards,
Guillaume