Open the Alexa app and tap the menu icon, if you don't have it 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"
At the bottom of the screen is a note about connecting your iPad or iPhone remember the name in the bottom of this screen.
Press your device home button and open up the iOS Settings app, then select Bluetooth
Select your Alexa device from the list with the same same that was displayed in your Alexa app.
Once paired, you can go ahead and close both the Settings app and the Alexa app.
If you have connected via Bluetooth to this device before, sometimes the iPad / iPhone won't connect to it again this is an iOS Bluetooth issue
Simply tap the "i" icon next to the name and choose "Forget this device". then rescan for Bluetooth devices and now select it. It should now pair for you.
In MusicStreamer your Alexa should already be selected as the playback device. If it isn't, simply tap the AirPlay icon to the right of the volume slider and select your Alexa from the list.