Instructions
Click on the "Dungeon Finder" icon. Select "Specific Dungeons" from the Type drop-down menu.
Check the "Ragefire Chasm" checkbox.
Select your role from the options at the top of the screen. You can be a Tank, Healer or DPS depending on your class and build. Click the "Find group" button.
Click the "Enter Dungeon" button when the Ragefire Chasm popup window appears. You will be teleported into the Ragefire Chasm instance.