I am not sure if you still need an answer to this, but there may be someone with the same problem at some point.
To set a RadioButton to the checked state from the designer or code, you have to set the IsChecked property.
In XAML that would be similar to the following:
<RadioButton x:Name="dumbAIRadioButton" Content="Dumb" HorizontalAlignment="Left" Margin="37,41,0,0" VerticalAlignment="Top" Checked="dumbAIRadioButton_Checked" IsChecked="True"/>
You would set that property to true on the one you want checked.