CreateUserWizardStep step = (CreateUserWizardStep) CreateUserWizard1.FindControl("CreateUserWizardStep1");
if (step!=null)
{
TextBox txt = (TextBox)step.ContentTemplateContainer.FindControl("txt_desc");
}
or if you can see your step control from server-code
TextBox txt = (TextBox)CreateUserWizardStep1.ContentTemplateContainer.FindControl("txt_desc");