On your photo booth, go to C:\Program Files (x86)\Photo Booth Solutions\Social Booth\Tools
The way it works is for each of the 4 bottom buttons, you can replace the text with something custom. The custom text is defined by the existence of and content of CustomButton1.txt, CustomButton2.txt,CustomButton3.txt,CustomButton4.txt
So if you put the included CustomButton1.txt in the same folder as Keyboard.dxr, the contents of the text file will replace the original 1st button.
You can replace all 4, or as many as you like. The text won't automatically resize, so keep it short.
If you want to add an @ symbol before all of the original buttons, you can define all 4 customButton.txt files with @aol.com, @hotmail.com, @gmail.com, @yahoo.com, etc.
Or you can switch the domain names if no one uses AOL, or even if you want to add a .edu button instead.