Open the Alexa app and tap the "menu"More" icon, if you don't have the Alexa app installed you can download it here.
Select Settings->Device Settings
Return to the Alexa app and tap 'Skip this step', this is an error in Alexa's app your device should now be paired to your Mac.
This will stream all the sounds from your Mac.
If you want to stream your entire music library or your playlists from your mac or another computer or a network media drive to your Alexa device then you can via MusicStreamer for Mac. Simply download the app, scan your network and pair to your Alexa device.