Use the large triangular parking lot to the left. The train station is just above and to the right. The train and station is on Dickerson St. (If this map doesn’t work any more, look for “50 S Morris St, Dover, NJ 07801”. Across the street to the west of it is a large parking lot. 50 yards to the north and then 20 yards east is the Dover train station.)
Note that most of the spaces are “Monthly Permit only”. Some of them are 12 hour meters. It’s like $3 for 12 hours. The lot closes at 10 or 11pm… They don’t close the gate but you might get a $30 parking ticket after that time.
If you plan on getting there late and staying late, you might be better off with street meter parking near the station… though I haven’t tried that.