Gridview Checkboxfield is grayed out! (ASP.Net 2.0/Visual Web Developer 2005)

I have created a datatable, added it to a dataset, and then bound it to a gridview.  The datatable has two columns: a string column named filename and a boolean column named include.  I've generated the columns in my gridview.  the code for it is here:

<asp:GridView ID="gvfiles" runat="server" AutoGenerateColumns="false" EmptyDataText="No Records found!">
    <Columns>
            <asp:TemplateField HeaderText="Filename">
            <ItemTemplate>
            <%#getfilename(Eval("filecolumn"))%>
            </ItemTemplate>
            </asp:TemplateField>
            <asp:CheckBoxField datafield="includecolumn" HeaderText="Include in Exception?" />
            </Columns>
 </asp:GridView>

When the gridview is displayed the checkbox is "grayed out" and unusable.  I want to be able to check the checkbox and change the dataset.  Is that possible?

Here is the code I use to create the dataset:

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Dim basepath As String = Request.QueryString("path")
        Dim filedataset As New dataset()
        Dim filedatatable = New DataTable("filetable")
        Dim filecolumn As New DataColumn("filecolumn")
        filedatatable.columns.add(filecolumn)
        Dim includecolumn As New DataColumn("includecolumn")
        filedatatable.columns.add(includecolumn)
        Dim filelist As String() = Directory.GetFiles(basepath)
        Dim filename As String

        For Each filename In filelist
            Dim filetablerow As DataRow = filedatatable.newrow()
            filetablerow.Item(filecolumn) = filename
            filetablerow.Item(includecolumn) = "false"
            filedatatable.rows.add(filetablerow)
        Next

        filedataset.Tables.Add(filedatatable)
        gvfiles.DataSource = filedataset
        gvfiles.DataBind()

    End Sub

Thanks in advance for your help!