Given Microsoft's stance about security and fragmentation, "unlocking" your WP device is a bit tricky. Here are the steps:
- Install Windows Phone SDK from here. 1GB+ download. Restart your computer after done.
- Connect your phone using USB cable.
- Make sure your phone's date and time are correct
- Go to Start Screen
- Type Windows Phone Developer Registration
- Follow the on screen wizard and login using your Microsoft account.
This will unlock your device so you can install "developer" apps.
How to install an app:
- Download WP8 Power Tools from here.
- Connect to your device
- Browse the XAP file
Although Windows Phone has one of the best youtube app of all platforms (MetroTube), the YouTube app from Microsoft is really nice too. Everyone knows Google's underhanded tactics against Windows Phone regarding the youtube app. They have blocked the youtube app's api key so the app doesn't work anymore.
Now there is a new youtube XAP that comes with a different key to workaround this issue. I have attached the XAP in here. After downloading, rename the .zip to .xap and follow the above process to install it!