You are not binding the key to a function, you are trying to bind it to an element which won't work.
bindKey ( "F2", "down",
function ( )
if ( not NitroWindow ) then
Gui ( )
end
guiSetVisible ( NitroWindow, true )
showCursor ( true )
guiSetInputEnabled ( true )
addEventHandler ( "onClientGUIClick", CloseWindowButton, close, false )
addEventHandler ( "onClientGUIClick", SetDefaultButton, default, false )
addEventHandler ( "onClientGUIClick", SaveChangesButton, accept, false )
guiScrollBarSetScrollPosition ( GreenScrollBar, GC )
guiScrollBarSetScrollPosition ( BlueScrollBar, BC )
end
)