My XAML file errors when I try <toolkit:AutoCompleteBox>
although I added a reference to the Microsoft.Phone.Controls.Toolkit.dll file. Everywhere I have read says this should work.
3 steps to make this work:
First reference the assembly in your project
Secondly reference the namespace in your xaml file
Finally point to the control you want to use in the namespace
I guess that you're missing the second step:
xmlns:toolkit="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone.Controls.Toolkit"