The VIN label is for the connector, the screw terminal, consisting of a + (VIN) and - (GND). With the limited space on the board for silkscreen the + is slightly obscured by the terminal (but we did put it on the bottom as well).
Anyway it sounds like you wired it correctly by following the wiring diagram and the silkscreen on the bottom: VIN = +'ve, GND = -'ve