You need to identify what kind of track your train set has.
If your track has three rails, like the track pieces on the left, you need to use the three-rail guide
If your track has two rails like the track on right, then continue reading here.
If you are using a Marklin System HO/AC powered set from Germany, the system is more like American three-rail systems than two-rail DC sets. The three-rail instructions will be more applicable, but pay close attention to the documentation that came with the set.