You really should read the manual ^^
3dgs is not like GameMaker. In 3dgs you have two options. One is to learn lite-c. The other option is using the templates. Using templates is like using GameMaker, you use already scripted stuff. So the examples above are working with 1000 levels if you want.

To use the code above u need a player loaded in a level. Before place a sprite to the pos in your map, where the level should be switched. Give this sprite the first action Door_To_Next_Room. Then place another sprite somewhere in the level and assign the action After_LevelLoad. If your player comes close to the first sprite a new map will be loaded. If this map contains a "After_LevelLoad" player will be set to the position. Hope u know what i mean.

Edit: These are just examples there are other ways to handle this.