You are in the wrong story range
Code:
Sub Podpisi()
Dim oRng As Range
Set oRng = ActiveDocument.StoryRanges(wdTextFrameStory)
With oRng.Find
.Execute FindText:="help", ReplaceWith:="Mr." & vbCr & "Adrian Tomc", _
Format:=True, Replace:=wdReplaceAll
End With
End Sub