The wait-for-signal is a systems specific command called from an event, not as an event. What this means is you need an event that sets the signal to be called as an action...
ie.. [This thing] On created-
action- Wait-for-signal (mySignal)
action- do this thing...
Then you have to send the signal as an action from another event.
ie.. Compare value {someVariable}=10-
action- Signal (mySignal)
The object waiting for the signal will now do this thing...
Now as imhotep22 pointed out, there are other ways to do this just as easy. Instead of waiting for signal you could simply put a variable (instance or global) that you changed values of and test for that to trigger the object or use a function call. Constuct has simply gave you a variable called signal and you are filling it with your tag and comparing its value with Signal command.