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
Then select your Echo from the list
Then Choose Bluetooth Devices.
Then tap the "Pair a New Device"
Open 'System Preferences' on you Mac and select 'Bluetooth'
Right click the 'Echo...' device listed in your Bluetooth connections and select 'Connect'
Once the Connection changes and displays the 'Connect' button, click this.
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.