Ok, I have read and followed the instructions as written in this link: http://en.support.wordpress.com/code/posting-source-code/
I have tried all other the solutions, including reinstall the whole wordpress site, installing syntax highlighting plugins etc. but it just din't work ?! Below is my published post in Twenty Thirteen. My post still display the code block as plain text as you can see.
Edit: I'm using self-hosted WP and installed (and activated) SyntaxHighlighter Evolved
It worked with single line code, but below is what it showed when I typed multiple lines of code: