For an experiment:
Q: How can I define an instruction for an experiment?
A: Right click the experiment you have created -> click "Experiment Instructions" -> fill in the instruction of experiment here
Q: Where the instruction of an experiment shows to me?
A: When you begin to run a subject, the experiment instruction pop up as a dialog window.

For a condition:
Q: How can I define an instruction for a condition?
A: Right click the condition you have created ->click "Properties" -> click "Stimuli" -> you will see the "Instruction" on the top as the first text box-> fill in the instruction of the condition here
Q: Where the instruction of a condition shows to me?
A: When you begin to run a trial in a subject, the instruction shows up on the top of the screen on software.