OK you can download the GSX installer from here
http://www.fsdreamteam.com/products_gsx.htmlOn the left click on "
Download FSX Version"
When it has downloaded just install it as normal
When you have installed it, open FSX and load into a flight.
Click on the "
Addons" menu at the top
Go down to "
Addon Manager"
In the new window on the left side make sure "
FSDreamTeam" is selected
In the box just to the right of that select "
GSX"
Then at the bottom click on "
Register Serial"
Enter here your serial number that you have in the email (you can copy and paste) and then click OK
You will be asked if you are sure and just click Yes or OK
You can also read the manual found here FSX Default Folder\fsdreamteam\couatl\gsx\GSX_manual.pdf