I have a user control that I've created, however when I go to add it to the XAML in the window, Intellisense doesn't pick it up, and I can't figure out how to add it to the window.
You need to add a reference inside the window tag. Something like:
xmlns:controls="clr-namespace:YourCustomNamespace.Controls;assembly=YourAssemblyName"
(When you add xmlns:controls=" intellisense should kick in to make this bit easier)
Then you can add the control with:
<controls:CustomControlClassName ..... />