If you have loaded a map at least once with this add-on installed, a settings.cfg file will be created into Left 4 Dead 2/left4dead2/ems/staggeringspecials

Here is the default code:
You may change the chance values to the corresponding special infected.

NOTES

• Make sure to put INTEGER values and do not put any other types, otherwise it will set the value automatically to 100 when invalid type is triggered.
  
• FLOAT or decimal types will turn into an integer.
  
• Putting values out of the boundary of 0 to 100 will clamp the value.

In case of other errors, you would have to load a map with the add-on again to gain the default code or just copy the code above.

The addon has functionality on all gamemodes and maps, but for PvP gamemodes like Versus it is automatically disabled by Valve.

To make it work in Versus on your local server, right-click on Left 4 Dead 2 in your steam library, select Properties, click on the Installed Files tab, then click on the Browse... button.
Go to left4dead2/cfg/ and open addonconfig.cfg with a text editor or Notepad.

Finally, edit this line:
"versus" "1"
to
"versus" "0"