When play starts, you see the results of your first throw of the dice.
You can only use each combination once, i.e. once you've entered something in a slot, you can't change the score later if you roll a better hand.
There is one exception to this rule, if you roll a yahtzee a second time you can put it in the yahtzee row again for an extra fifty points. You can keep doing this each time you get another yahtzee. At the end of the game you will be given extra rolls to fill in the other rows. Be warned that if you score zero in your yahtzee row (i.e. you filled that row with something that wasn't a yahtzee) then you can't put a yahtzee in afterward, zero is all you will score.
There is also a bonus of 35 points if you score a total of 63 points or more in the top six rows.
As an alternative to using the mouse, the keys 1 through 5 toggle the dice just as if you had clicked them. The key 1 corresponds to the first die and so on. Use the usual controls to roll the dice and select a row. i.e. use tab to switch between the roll button and the score area, use the arrow keys to navigate up and down the rows and use space to select.