Private Sub frmFiles_Load(sender As System.Object, _
        e As System.EventArgs) Handles MyBase.Load
 
        dt.Columns.Add("Datei", GetType(String))
        dt.Rows.Add("C:\daten\demo.txt")
        With dgv
            .AutoGenerateColumns = False
            .DataSource = bs
            Dim dc As New DataGridViewButtonColumn
            .Columns.Add(dc)
            dc.DataPropertyName = "Datei"
        End With
    End Sub

 Private Sub dgv_CellMouseClick(ByVal sender As Object, _
        ByVal e As System.Windows.Forms.DataGridViewCellMouseEventArgs) _
        Handles DataGridView1.CellMouseClick
 
        If DataGridView1.Columns(e.ColumnIndex).DataPropertyName = "File" Then
            Dim cell As DataGridViewCell = _
             DataGridView1.Rows(e.RowIndex).Cells(e.ColumnIndex)
 
            If Not IsDBNull(cell.Value) Then
                Dim filename As String = _
                    CType(cell.Value, String)
                'Vorhandene Datzei öffnen 
                If IO.File.Exists(filename) Then
                    Process.Start(filename)
                End If
 
            Else
                'neue Zeile anlegen und per Dialog füllen
                If OpenFileDialog1.ShowDialog = _
                  Windows.Forms.DialogResult.Cancel Then _
                   Exit Sub
                dt.Rows.Add(OpenFileDialog1.FileName)
            End If
        End If
    End Sub

dt.Rows.Add(OpenFileDialog1.FileName)