Try this:
Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs) Handles ComboBox1.SelectedIndexChanged
Dim WantedValue As Integer = 0
For Each dr As DataGridViewRow In DataGridView1.Rows
WantedValue = WantedValue + CInt(dr.("AccessTableColumnNameYouWant").value.tostring)
Next
TextBox1.Text = WantedValue.ToString
End sub