Even though I have downloaded the emulator, I can't start it

I downloaded the emulator and I have restarted my mac but when I open a project and click on emulator, it says wait. I have waited like ten or more minutes but it still doesn't open. When I open the folder called emulator it gives bunch of stuff. I clicked on emulator and it says this
emulator: ERROR: No AVD specified. Use '@foo' or '-avd foo' to launch a virtual device named 'foo'
I don't know what to do.