When you do a replace operation, you need to provide a value that you are searching for, a value that you want to replace it with. Then you need to press the search button. When the searched for phrase is found, you can press the replace button, or the all button if you want to replace that instance or all instances. If you don’t want to replace the current instance, you can press the search button again and ME will look for the next instance, without making the replacement.
Hope this helps,