It would be much simpler to define a style that includes a properly
formatted frame
(http://sbarnhill.mvps.org/WordFAQs/MarginalText.htm#TrueMarginalText). The
style can be assigned a toolbar button or keyboard shortcut to make it easy
to apply, no macro needed.

If for some reason that doesn't do what you need, then look at the VBA help
for the AddTextbox method of the Shapes collection object, something like

   Dim myBox As Shape
   Set myBox = ActiveDocument.Shapes.AddTextbox( _
       Orientation:=msoTextOrientationHorizontal, _
       Top:=12, Left:=-72, _
       Height:=72, Width:=64, _
       Anchor:=Selection.Range)
   With myBox
       .Fill.Visible = msoFalse
       .Line.Visible = msoFalse
       .TextFrame.TextRange.Text = "hello"
   End With

You'll need to work with this to get the vertical position (.Top) correct,
because the measurement is from the top line of the current paragraph, not
from the cursor position.