IWPFieldContents is passed as untyped "object", so please create a variable of the type IWPFieldContents and assign it:
Private Sub WPDLLInt1_OnFieldGetText(
ByVal Editor As Long,
ByVal FieldName As String,
ByVal Contents As Object)
Dim theContents As WPTDynInt.IWPFieldContents
Set theContents = Contents
If FieldName = "name" Then
theContents.stringvalue = "Julian Ziersch"
End If