testfor command

for the testfor command to activate the comparator

the test for command in the command block did nothing to the command

1.i made a redstone clock with a command block with the command testfor @p[score_zombie_min=1] (zombie was the name of my score count)
2. i activated the clock, the clock worked but nothing happened with the comparator

i used then set up that can be found near the end of http://www.youtube.com/watch?v=Bdd_2taSpG8 this video
i was playing in single player