That looks correct. In case you're interested there is a very complete discussion of robots.txt here
tools.seobook.com/robots-txt/ Google meta descriptions are completely automated by Google. They will use the meta description unless they think they can do a better job. So you need your meta description tag, and you also need some text on the page.
The description may not be present until the page is crawled. It may take a while if you've made page changes recently or just posted. Here's a checklist of things you can do to try to make sure you've done everything.
Check this all first:
1. Did you put all meta title, description on important pages?
2. Is your meta tag syntax correct?
3. Did you already edit,save sitemaps with new pages that you want to get crawled?
4. Did you resubmit sitemaps to Google webmaster tools? Remember refresh it/resubmit it and remove the old one.
Here's a page from google that tells you how they get the description and what you might need to do to make sure it's getting the notice it deserves.
support.google.com/webmasters/bin/answer.py?hl=en&answer=35624 This isn't something that is caused by Joomla as far as I can tell. Once a page shows up on the internet, it's up to Google to get the description.
I hope that answers your question.
Cheers,
Ed