If you're talking about Adobe Flash Player Standalone, right-click a .swf or .flv file from your computer and from the Open with list select Adobe Flash Player Standalone.
If you're using Adobe Flash Player, open your browser add-ons menu and enable the plugin. In Opera, you will need to navigate to Tools > Extensions while in Mozilla Firefox, you will need to press Ctrl + Shift + A key combination.
If you're using Internet Explorer, you can open the add-ons menu if you press Manage Add-ons from the Tool menu. In Google Chrome, the player is installed and enabled by default.