C:\Users\yourusername\AppData\Roaming\Virtuali\Airplanes.

In that folder, if they are not already there, you have to create 2 new folders

\CS_B757-200-III_PW2037
\CS_B757-200-III_PW2040

Take the gsx files for each variant and just drop them into their respective folders. then rename each one to"gsx.cfg". Click ok if you get a windows caution about changing the file extension and thats it.  I would post images of the results but the forum doesn't allow me.