It's a long time since I last played RHG but let's see what I can remember. The easter secret level can be found in a level called "Turtle's edge". The trick is to shoot the bonus post hidden somewhere near the "Let's just say SECRET !" post. However there is also a trigger preventing you from shooting it the first time you enter the cave. All you have to do is leave the cave and reenter it. Stand exactly where the "Let's just say SECRET !" post is and shoot your blaster to the right. That's all.