Download Source SDK Base 2007:
Go to your Steam Library and click the drop down menu next to the search box under the word LIBRARY. It probably says ALL GAMES for you. Select Tools and look for Source SDK Base 2007 in the list. Right-click and select Install Game....

Run Source SDK Base 2007 once.
Let it load until you see the main menu, then click Quit. Some people need to do this to be able to run IOSoccer, so just run it to be on the safe side.

Download the game installer.
https://dl.dropboxusercontent.com/u/14644518/IOSoccer_Setup.exe

Restart Steam. The mod should now show up in your game list.

To make sure that IOSoccer is in the folder check if the path to your iosoccer folder looks similar to this: X:\Program Files\Steam\steamapps\sourcemods\iosoccer.