It should "just work" without doing anything extra. If it doesn't, either you're not set up right, or the site is down.
Here's one that I just tested to be working (hidden wiki):
http://wikitjerrta4qgz4.onion/
One other possibility, odd though it sounds, is that your clock is wrong. Tor needs an accurate system clock to coordinate with the network, and this is especially important for hidden services. Make sure both the time and time zone are correct.