Thanks.
Using your demo, single key presses still didn't work. So I mashed a whole lot of keys at once and got some results. Then went back and tried my own scene again.
Sure enough mashing many keys at once will register one or two keys. A single press does nothing. The same results in your scene and mine, so I'm using it right.
Weird. It's as if it's taking the input and then clearing it again in a single frame, too fast to see. And many simultaneous keys clog it up and the last few pressed will be kept. That's my novice explanation.
I got the same results in Playmaker 1.6.0 and 1.7.5 and in Unity 3.5.7, 4.0.0, and 4.2.0, all in Windows 7 64Bit.