If i understand you correct, you mean the basic part where you set the max time and score. As you guessed this goes into the description.ext
The code looks as the following:
titleParam1 = "Time:";
valuesParam1[] = {100000, 300, 600, 900, 1200, 1500, 1800, 2700, 3600, 7200};
defValueParam1 = 1800;
textsParam1[] = {"Unlimited", "5 min", "10 min", "15 min", "20 min", "25 min", "30 min", "45 min", "1 hour", "2 hours"};
titleParam2 = "Score to win:";
valuesParam2[] = {10000, 5, 7, 10, 15, 20, 25, 30};
defValueParam2 = 5;
textsParam2[] = {"Unlimited", 5, 7, 10, 15, 20, 25, 30};
To end the game just make a trigger with the following in it's condition field:
time >= param1
You might also make a timer script which is, as i heard, a little more accurate than the "time" command. But in general it should be accurate enough ;D