Where you have them is just fine. Sorry, I guess I'm going blind here. Or at least a little nuts.
Try adding what I've highlighted in RED:
Code:
Private Sub CboType_NotInList(NewData As String, Response As Integer)
Response = acDataErrContinue
If MsgBox("Sample Type " & NewData & " is not in list. Add it?", vbYesNo) = vbYes Then
Dim db As DAO.Database
Dim rstType As DAO.Recordset
Dim sqlType As String
Set db = CurrentDb()
sqlType = "Select * From Sample_Type"
Set rstType = db.OpenRecordset(sqlType, dbOpenDynaset)
rstType.AddNew
rstType![Type] = NewData
rstType.Update
Response = acDataErrAdded
rstType.Close
Set db = Nothing
Set rstType = Nothing
End If
End Sub