You mean once it's installed? Just click the icon. Then, on the first part, select what Trainz version you are using. After that, click 'Open' and a list of routes should show up. Pick the one you want to edit and click 'Ok'. If there's a specific function you need help on, let me know, I may be able to help.
PS: Thank you all who have helped me with TrainzMap, I've changed the track on MRL and it works fine!