Accessing SQL CE from WP7 Mango error

I'm running through the sql access example on the MSDN website (http://msdn.microsoft.com/en-us/library/hh202876(v=VS.92).aspx) and i can't understand why the following code isn't working.

MSDN Example: http://msdn.micro开发者_运维百科soft.com/en-us/library/hh202876(v=VS.92).aspx

My Code: http://pastebin.com/MxWtGvPw

The Error: The type or namespace name 'ObservableCollection' could not be found (are you missing a using directive or an assembly reference?)

I'm really struggling to get my head around this so if anyone has any pointers, that would be great.

Thanks!

You seem to be missing:

using System.Collections.ObjectModel;