First things first you are missing a big chunk of the header:
xml code:
>>><?xml version="1.0" encoding="UTF-16"?><<<
>>><Interfaces xmlns="urn:msgplus:interface" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="urn:msgplus:interface PlusInterface.xsd"><<<
<Window Id="WndTest" Version="1">
<Attributes>
<Caption>Test Window</Caption>
</Attributes>
<TitleBar>
<Title><Text>Hello!</Text></Title>
</TitleBar>
<Position ClientWidth="170" ClientHeight="45"/>
<DialogTmpl/>
<Controls>
<Control xsi:type="StaticControl" Id="LblTop">
<Position Left="10" Top="10" Width="150"/>
<Caption>Hello world!</Caption>
</Control>
<Control xsi:type="ButtonControl" Id="BtnClose">
<Position Left="115" Top="25" Width="50"/>
<Caption>Close</Caption>
</Control>
</Controls>
</Window>
</Interfaces>
/ 
