"When I entered a new calf and tried to use the Guess Sire option, nothing happened."

If you are using natural service as your primary breeding option and use the Turn in Bulls option in CattleMax, the date the bull or bulls are turned into a pasture with the cows will be used as the "Exposed" date.  The estimated date of birth of a cow's calf will be the date the cow is bred (AI or ET) or the bull(s) is (are)  turned into the pasture plus the Average Dam Gestation Days (months) entered under Settings/Preferences/Herd Adjustments.  For the Guess Sire option to work, the calf must be born within a 67-day window - from 30 days prior to the estimated date of birth to 37 days after the estimated date of birth.  If the calf is born outside that window, the Guess Sire option will not work.