There are mulitple ways to enter a racer's finishes. A finish can be entered into the racers record by clicking on any finish cell. (If you hover over a finish cell the 'Race number' will pop up.) You can also enter a racer's name or sail number in sequence - 1st finisher, then 2nd finisher, etc.
Enter directly into a racer's finish cell
Find the racer's record, click on the appropriate finish cell, then enter the finish or penalty (see Entering Penalties). The finish can be entered in the finish cell on the racer's line, or in the drop down Update Racer Details (click the down arrow), both shown below...