@CarlosLanderas is correct depending on where you've placed the DIV control. The DIV by the way is not technically an ASP control, which is why you cannot find it directly like other controls. But the best way around this is to turn it into an ASP control.
Use asp:Panel instead. It is rendered into a <div>
tag anyway...
<asp:Panel id="divSubmitted" runat="server" style="text-align:center" visible="false">
<asp:Label ID="labSubmitted" runat="server" Text="Roll Call Submitted"></asp:Label>
</asp:Panel>
And in code behind, simply find the Panel control as per normal...
Panel DivCtl1 = (Panel)gvRollCall.FooterRow.FindControl("divSubmitted");
if (DivCtl1 != null)
DivCtl1.Visible = true;
Please note that I've used FooterRow, as my "psuedo div" is inside the footer row of a Gridview control.
Good coding!