Javascript code:
function OnWndMenuEvent_CtrlClicked(Wnd, ControlId){
if(ControlId == "BtnLetters")
{
Wnd = MsgPlus.CreateWnd("Letters.xml", "WndLetters"); }
}
You are using Wnd twice that isn't the problem but that is a no no.
Does the window open at all? If not what shows in the debug window?