If using WP7:
You need to add the CredentialsProvider attribute to the XAML:
<my2:Map HorizontalAlignment="Left" Margin="14,23,0,0"
Name="map2" VerticalAlignment="Top"
Loaded="map2_Loaded"
Height="534"
Width="436"
CredentialsProvider=
"Ahk5en93JIJlP4ObG_CqLoLgWU22jhPFJbe52bOTjw_hYxB3EtthoVL"
/>
The key above is taken from the maps portal website. See this guide for more info:
http://www.c-sharpcorner.com/UploadFile/raj1979/how-to-use-the-bing-map-control-in-windows-phone-7/
If using WP8:
See here, this property is no longer used as the functionality that you use the API key for is built into the SDK, so no need to make any calls to MS servers.
CredentialsProvider for the bing map in windows phone 8 application