You can do as suggest above... keep modifying the efficiency number until the OG matches what you measured, but you can also just click on the ANALYSIS tab and enter your measured OG.
Now, getting a bit more specific...
If you're measuring preboil gravity, make sure the preboil "kettle" volume that the software says you should have is actually what you DO have. Then hold the mouse over that field (kettle volume) and it will pop up a window that shows you what the preboil gravity should be (if you're on track for your expected efficiency that is). If it doesn't match, adjust the efficiency until it does.
If you're done with the boil and want to measure brewhouse efficiency, make sure your fermenter volume accurately matches the finished batch size in the software (adjust if necessary), then go to the analysis tab and over-write your OG. Now it will tell you your brewhouse efficiency.