Instructions
Insert a downloaded "StarCraft II" replay file into the game's replay directory: My Documents/StarCraft II/Account/(your account number)/(more specific numbers)/replays. You can simply drag and drop the file into the folder or you may copy/paste.
Start "StarCraft II" by inserting the game's disc or double-clicking the desktop icon. The game will boot to its main menu.
Log in to your Battle.net account.
Click the replay button at the bottom of the screen. The button is blue and resembles a small camera. Your replays, including any downloaded replays from the game's beta phase, will be included in the menu.
Highlight the replay you want to play by left-clicking on it.
Press the "Play" button to begin playing the replay.