1. Put the map in your Portal 2's maps folder (Default: C:\Program Files\Steam\steamapps\common\portal 2\portal2\maps)
2. Launch Portal 2
3. Go to Options > Keyboard/Mouse
4. Enable "Allow Developer Console"
5. Go back to the main menu, press the "~" key
6. Type "map mortal_dream" without the quotation marks in the text field.