Adding Markers
There are several ways to add markers.  

  Click the Add Marker above the tracks and to the left of the time line.  The marker will be added at the time offset of the playback indicator.  If snapping is on, it will adjust based on the Snap Settings.

You can also double click the time line to add a marker or right click on the time line and select Add Marker... Other methods of adding markers include clicking the Mix menu followed by clicking Markers followed by clicking Add Marker... or by clicking the Mix menu followed by Add Tempo / Key Change.

Markers obey the current Snap Settings.  Markers with time signature changes always snap to the measure, as it doesn't make a lot of sense to add a time signature change in the middle of a measure.