Copy sp_edifice.bsp to Steam\Steamapps\common\portal 2\portal2\maps.

You will find the Steam folder at:
Windows 32-bit: C:\Program Files\
Windows 64-bit: C:\Program Files (x86)\
Mac: macusername/Library/Application Support/

After you have copied the bsp, open the console in Portal 2 (usually with the ~/` key) and enter "map sp_edifice".