Why doesn't c# find the attribute? asp.net

Because the Attributes collection doesn't work that way:

Gets the collection of arbitrary attributes (for rendering only) that do not correspond to properties on the control.

If you want to have properties like that, you need to create your own custom control that has the properties you want. Or, as an alternative, create a UserControl that hosts a single CheckBox and associated div or whatnot -- then you can just reference the one related div by ID in the codebehind. Instantiate multiple instances of that control, and you're good to go.

Edit: my WebForms-fu is a bit rusty, but here goes nothing.

The control class:

using System;
using System.Web.UI;
using System.Web.UI.WebControls;
namespace UserControlExample {
    [ParseChildren(false)]
    public class TogglePanel : UserControl {
        private CheckBox cbToggleContent = new CheckBox();
        private Panel pnlContentPlaceholder = new Panel();

        public TogglePanel() {
            Load += OnLoad;
        }
        public bool Checked { get; set; }

        private void OnLoad(object sender, EventArgs eventArgs) {
            Controls.Add(cbToggleContent);
            Controls.Add(pnlContentPlaceholder);

            if (!IsPostBack) {
                cbToggleContent.Checked = Checked;
                pnlContentPlaceholder.Visible = Checked;
            }

            cbToggleContent.AutoPostBack = true;
            cbToggleContent.CheckedChanged += (s, args) => {
                pnlContentPlaceholder.Visible = cbToggleContent.Checked;
            };
        }

        protected override void AddParsedSubObject(object obj) {
            pnlContentPlaceholder.Controls.Add((Control) obj);
        }
    }
}

And its usage:

<%@ Register TagPrefix="a" Namespace="UserControlExample" Assembly="UserControlExample" %>

<a:TogglePanel Checked="True" runat="server">
    This stuff here will be shown or hidden based on the checkbox
</a:TogglePanel>

FindControl only searches the current naming context and does not traverse a hierarchy. By calling FindControl they way you are, it is using this.FindControl. Try something like chk.Parent.FindControl(...) if the div is a sibling of the CheckBox

Edit:

Ah, well, the Attributes collection is "for rendering purposes only". It doesn't appear to be populated with attributes specified in the ASPX HTML declaration.

http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.webcontrol.attributes.aspx

Off the top of my head - doesn't the checkbox have an InputAttributes collection?

Admittedly, an untested stab:

CheckBox chk = (CheckBox)(sender);
object div = FindControl(chk.InputAttributes["myDiv"]);

Does something like that work?