You compiled your code behind into a dll, does that dll exist in the Web/bin folder?
Make sure your post build event copies the dll up to Web/bin
You also have a .ascx file as part of your usercontrol right?
For best results you should set a reference to the mojoPortal.Web.UI project using a project reference. Then in your custom module you should inherit from SiteModuleControl instead of UserControl, though it can work with a plain UserControl.
Hope it helps,
Joe