Might be useful to know what device you are talking about. Essentially you go to the position you want to mark a lap point, and in the case of my edge500, press the lap button. You might need to drill down to the setup menu to set it up to behave like you want. Unfortunately, these lap points go away after you reset the timer, and you have to set them all over again for the next time you are on that route.
If you are specifically asking about the 'mark and lap' feature in setup, I think that just allows you to set up your lap points with out having the timer going. You still set your lap points by pressing the lap button.
