Problem with Sessions

[Kolags]

Hi All,
I have a datagrid with check box in each row at first column; our requirement is looks like hot mail and yahoo mail working system, after selected mail(s) using check box user can delete mail. In the same way, we would also develop functionality for edit, delete buttons only when checked row from the datagrid.
I have started like this, whenever I click on the edit button after I selected check box(s), I need to open a popup window only for the checked rows, for this I have created a session in first page and passing the parameters to second page (Popup).

Edit button
Dim rowCount As Integer = 0
Dim mySelection As StringBuilder = New StringBuilder
Dim DemoGridItem As DataGridItem
For Each DemoGridItem In DGWHPkg.Items
Dim myCheckbox As checkbox=CType(DemoGridItem.Cells(0).Controls(1, CheckBox)
If myCheckbox.Checked = True Then
rowCount += 1
mySelection.AppendFormat("'" & "{0}" & "'" & ",", DemoGridItem.Cells(2).Text)
End If
Next
lblChosen.Text = mySelection.ToString().Substring(0, mySelection. ToString().Length - 1)
Session.Add ("Chosen", lblChosen.Text)

Popup window
If Not Page.IsPostBack Then
Dim cScript As String
cScript = "<script language='javascript'> opener.location.href = 'WebForm4.aspx'; </script>"
RegisterStartupScript("parentwindow", cScript)
lblChosen.Text = Session.Item("Chosen")

Now the problem is only few times the session is calling from main page to child (popup) page.
How to add the session to next page, whenever the edit button clicked?
But I am unable to do it. Please suggest me how to solve it. Guidance for Delete button too.