You put in the URL to be linked, then one single space, followed by the linking text enclosed in (parenthesis), like so.

If you're replying to another user, you can just put an "@" symbol before their *user* name (in my case, 'ipisdei'), not their screen name.

oh! i was having a hard time with the user name bit. thank you so much!

Actually, there's also a drop down menu, if you hover over a person's name. This will give you an option to reply directly to their comment.

ooo and it does the @ (name) thing automatically! i feel like quite an idiot now heh

Full disclosure: I only found out about the menu option yesterday. I was using the "@" method because that's how it works in Twitter as well