For each line, use Split to break the line into fields. Total the desired field.

Private Sub cmdGo_Click()
Dim file_num As Integer
Dim txt As String
Dim lines() As String
Dim i As Long
Dim fields() As String
Dim delimiter As String
Dim field_num As Integer
Dim total As Long

    delimiter = txtDelimiter.Text
    field_num = txtField.Text

    ' Read the file.
    file_num = FreeFile
    Open txtFilename.Text For Input As #file_num
    txt = Input(LOF(file_num), file_num)
    Close #file_num

    ' Process the lines.
    lstValues.Clear
    lines = Split(txt, vbCrLf)
    For i = LBound(lines) To UBound(lines)
        ' Break the line into fields.
        fields = Split(lines(i), delimiter)
        If UBound(fields) >= field_num Then
            total = total + CLng(fields(field_num))
        End If

        ' Display the value.
        lstValues.AddItem lines(i)
    Next i

    ' Display the total.
    MsgBox "Total: " & Format$(total)
End Sub