For model PSP-2001 and PSP-3001

Go to (Network) > (Internet Browser) and press the button.

Press the button to bring up the Internet Browser menu.

Navigate to [Tools] > [Settings] > [View Settings]

Go to "Flash" and press the button, then move up or down with the direction pad to select "On".
For model PSP-1001

Go to (Settings) > (System Settings) > [Enable Flash Player] and press the button.
You must connect to the Internet before you may enable the Flash player.