It's more limited for free accounts - paid accounts get full-text search by journal and full-text search of comments; free can only search the whole site and no comments. It's under "explore - site and journal search". You can still search your own entries by searching your username + your search terms on full-site search, though.

There's a weird message there saying it's blocked for my account type if I try to use it while on a free account, but it still works, so I think the message has just gone astray.

It only searches public posts, and it only searches people who have chosen the option to make their posts available in search, so it's not comprehensive, but it's so helpful for people who are bad at tagging well.