Greetings and Welcome.
The final gravity must be subtracted from the Original gravity. The difference is used to determine the alcohol content.
Within BeerSmith, there is both a tool for this process, plus it's automatically calculated in your recipe if you log the data as you go. Any field that says "measured" is your cue to add the appropriate information. By default, there is some junk... er, "placeholder," information in these fields, already. Just replace it with your actual measurements.