My friend sent me a .unity file. How don't know how to open it, I have a Mac and nothing happens when I import it. Thanks.
A .unity file is a Unity 3D Scene file. This is the core file and you can open it with Unity3D (indy, or any other version.)
However, just a Unity file on it's own is not much use, your friend needs to send along the entire Unity project, which is easiest done by zipping up the project's root folder and sending it over in its entirety.
Actually exporting as project probably would also work