Click the little chain link icon on a post you want to link to, then copy the text from the box that pops up. Then, when writing a post, click the little chain link icon in the toolbar above the edit box, and paste in your link. You can type in the text you want it to display as in the “optional title” box, or you can highlight some text in the edit window before you click the chain link button to turn it into a link to whatever you paste in the link box.
Here’s a link to @Studs’ thread, and one to @ogunther’s .