The Message Center can be found within the free iHeartRadio App for iOS and Android devices-Please note that the Message Center is NOT ACCESSIBLE Via Web. 

Simply open the app, log into your iHeartRadio account, and tap on the gear in the upper left corner of the screen to open the Settings menu (shown below).

From there, tap on the Message icon (shown below) in the upper right to access the Message Center to access any messages.