Getting Control Inside Server Control

I have a server control on my aspx page. in the code behind, i add a series of checkboxs to the control. the problem comes that I cannot get the elementID in java script. below is my code:

aspx:

Code:
<div runat="server" id="theContainer" style="position:relative; border:1px solid grey; width:51em; height:25em; overflow:auto; ">
</div>
<script type="text/javascript">

[Code] ....

code behind:

Code:
Dim myDiv As HtmlGenericControl
Dim sb As New StringBuilder
myDiv = New HtmlGenericControl("DIV")
myDiv = FindControl("theContainer")
sb.Append("<div style=""position:relative; top:50%; left:50%;visibility:hidden;"" id=""notFound"" >No Results Found</div>" & vbCrLf)

[Code] ....