To Add namespaces to the XAML file, go to the top root like Window or UserControl etc and add the desired namespace.
Writing the 'xmlns:xyz=' will display the available namepasces, if its not there then you have to add the corresponding assembly.
Here 'xyz' acts like a key to access the classes present in this namespace.
Like
BTW, NameSpace System.ComponentModel is present in WindowsBase assembly.
http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:compModel="clr-namespace:System.ComponentModel;assembly=WindowsBase"
Title="Window1" Height="300" Width="300">