In responding to the prompt about a time when you walked away from Omleas, I think you should think about a time when you turned away from a "happy" situation because you knew of the unhappiness it caused to another.
With any question like this, it's important to go back to your teacher to clarify their thinking. Given the way you have framed it in the question, you should think about a time when you have walked away from being happy upon realizing that it caused someone's pain. The people who walk away from Omelas leave because they cannot accept the premise that their happiness is based upon the unhappiness of the child. Le Guin writes that these people leave once they see the child:
At times one of the adolescent girls or boys who go to see the child does not go home to weep or rage, does not, in fact, go home at all. Sometimes also a man or woman much older falls silent for a day or two, and then leaves home. These people go out into the street, and walk down the street alone. ... They go on. They leave Omelas, they walk ahead into the darkness, and they do not come back... But they seem to know where they are going, the ones who walk away from Omelas.
The people "walk down the street alone" because they have seen the child and know it's suffering. In your case, think about a time when you could not remain with a group knowing that the group's happiness is because someone else is in pain. For example, consider a time when you were in a group of people who were laughing about another person or if someone was being taunted while everyone else was happy about it. If you walked away from that setting, you would be "walking away from Omelas." You walked away because you could not tolerate you/your group's happiness resulting as a consequence of another person's pain. Another instance would be if you could not participate in something that everyone else was doing because you realized the truth behind it. For example, you could not purchase a particular brand of clothing once you realized the poor working conditions in which it was made.
In both situations, your decision to "walk away" was because you felt that the happiness of all people was not maximized, and you were not happy knowing it. You could not "come back" because you knew the truth that someone's happiness comes at the cost of someone else's pain.