Power On an Arduino board.

Please specify in the documentation where must be the + polarity on the Power Jack.

Regards.

You mean like this:

External (non-USB) power can come either from an AC-to-DC adapter (wall-wart) or battery. The adapter can be connected by plugging a 2.1mm center-positive plug into the board's power jack. Leads from a battery can be inserted in the Gnd and Vin pin headers of the POWER connector.

And how is this a software issue?