It's in the triggers in the Flag section. Directly after Flag 128 it should say 'Level Score' for MPD maps. Simply create that trigger, set it to 50.000 and then add an Action for 'Win Game' if thr script tree is under Player 1 or do something like Flag 1=1 and then add the script 'If Flag 1=1" to the Player followed by a 'Win Game' action.