The following will put the bookmark in the main header of the first section after the third character if there is one. (If you want it to start from page 2, then you will either need a section break and make it Sections(2) or have a different first page header.
Code:
Sub addBM_Header()
Dim oRng As Range
Set oRng = ActiveDocument.Sections(1).Headers(wdHeaderFooterPrimary).Range
If Len(oRng) > 3 Then
oRng.Start = oRng.Start + 3
oRng.Collapse 1
End If
ActiveDocument.Bookmarks.Add Range:=oRng, Name:="NombreEncab"
End Sub