Converting a .sx2 in to a .zip file
Some Singles 2 games doesn't seems to work well with .sx2 file in the "Extentions" sub folder. Others seems to accept only .zip files (yes, Singles 2 works with .zip files). If you're having problems seeing the add-ons you put in the "extensions" folder, maybe this is your solutions.
I'm using Winrar here but I imagine that Winzip should work the same. I'm using the "Shirtsammlung" add-on for that example.
Create a temporary folder on your desktop and name it "1"
Extract your zip file (the one with your .sx2 file inside) on your desktop.
Right-click on the .sx2 file and choose "Open with..."
Choose Winrar (Winzip)
Extract it in your "1" folder on your desktop
Verify if it's all there : a few folders plus a info2.txt file
Open the folder and select all files
Right-click on "Add to archive..." Next...

Rename the file with the good name (Shirtsammlung).
Click OK, a .zip file will be creat.
Cut / Paste this .zip file into your "extensions" sub folder. Close everything, open the game and try it Also read : Converting a .zip file in to a .sx2 file