Moving Markers
There are two ways to move a marker.

Drag A Marker
Click on the top of a marker on the time line and drag it to the left or right to reposition it.  It will obey the snap settings when dragging it.

Edit The Exact Time
You can edit a marker by double clicking it or by right clicking it and choosing Edit...  Alternatively, you can click the Mix menu, followed by Markers, followed by Edit, followed by the marker you'd like to edit.  On the Edit Marker dialog (shown below), adjust the offset time of the marker.

Use the up and down arrows next to the minutes, seconds or milliseconds for the marker to adjust the time offset.  Click the green play button to play the mix from this offset.

Note that you cannot adjust the offset so that it conflicts with another marker.