You only need to use x:key in resources , You don't need to use x:Name="BackgroundConvert".
<phone:JumpListItemBackgroundConverter x:Key="BackgroundConverter" Enabled="#FFA20025"/>
Then you can access it from Resources using it's x:Key="BackgroundConverter"
value in your code behind. Resources is a Dictionary.
var converter = (Microsoft.Phone.Controls.JumpListItemBackgroundConverter)this.Resources["BackgroundConverter"];