The NumericUpDown should lose the focus in order to add the CustomUnit to the value. Try to add a single TextBox below the NumericUpDown, type a value and click inside the text box - CustomUnit should appear. When click back into the input field of the NumericUpDown the CustomUnit will hide again while editing the value.
Hope this helps.
TRY TELERIK'S NEWEST PRODUCT - EQATEC APPLICATION ANALYTICS for SILVERLIGHT.
Learn what features your users use (or don't use) in your application. Know your audience. Target it better. Develop wisely.
Sign up for Free application insights >>