>It doesn't show up on the page. If I just add a plain old TextBox instead of a UserControl, it works fine.
>
>Any ideas what I might be missing?

Here is some code from the IBuySpy sample application for dynamically loading UserControls onto a web page. This is a great sample app for learning how to dynamically load usercontrols.

LeftPane.Controls.Add(Page.LoadControl("~/DesktopModules/SignIn.ascx"));

You should be putting this into the Page_Init event handler, not the Page_Load event handler because of the way that the events fire.

You can debug your aspx pages by adding trace=true at the top and it will show you all the control hierarchy at runtime on your page. Using trace.write will output debug statements at the bottom of the page.