Just enter the results in the spaces, and the app will count the score automatically. No need to check boxes for exact matches or anything. In order to discard spaces, enter a 0.
Every player is supposed to have a different scoring sheets. These sheets don't have the letters. The number layout is the same as in the printed score sheets, but the colors are randomly assigned. This gives you more than 100 trillion different score sheets, so it's fairly unlikely you'll ever see the same one twice. If you don't like your sheet, just reload to get a new one. Same goes for resetting: just reload.
Twenty One is designed by Steffen Benndorf and Reinhard Staupe and published by Nürnberger-Spielkarten-Verlag. This scoring device is created by Mikko Saari (firstname.lastname@example.org). Feel free to copy this (everything is included in the one HTML page: all functionality and images), but do buy the game if you like this.