1- After you download the Spring Update , you get this error message :
"Windows Live Messenger is not available from a Windows Live Child Account"
This happens when you try use the new Messenger feature - available on the Xbox 360 after the Spring Update - using child account or graduated account ( user created it before his age is 18 and now he is over 18 )
I checked this with Xbox customer support and they said they are aware of this issue and they are still investigating it.
In the mean time all you can do is create new gamertag with new Windows Live ID, but make sure the age of the account user is over 18.
2- Can't find the display name in the Xbox 360 dashboard.
If you can't find your display name in the Xbox 360 dashboard, just go to this link
Sign in to your messenger and change the display name to the one you want to be displayed. Then sign out from the messenger and sign in again . The issue will be resolved.