We’re not familiar enough with Joomla to help you too far with this, but make sure that your CMS allows for you to enter both arbitrary HTML and arbitrary Javascript – the Javascript include is what turns the simple blockquote markup into a full-fledged tweet embed. I imagine the SCRIPT tag you’re including as part of the embed is being stripped away somehow. If you include the request to widgets.js in the core HTML markup for your site (layout), then you’ll typically only need to include it once.
