To pad on the right, add the pad string to the right of the original value and use Left$ to trim the result.

Private Sub cmdPad_Click()
Dim value As String
Dim pad_text As String
Dim length As Integer

    value = txtString.Text
    pad_text = txtPad.Text
    length = CInt(txtLength.Text)

    If optSide(0).value Then
        ' Pad on left.
        txtResult.Text = Right$( _
            String(length, pad_text) & value, length)
    Else
        ' Pad on right.
        txtResult.Text = Left$( _
            value & String(length, pad_text), length)
    End If
End Sub