Either:
1) Use the [link|http://z.iwethey.org/forums/render/content/show?contentid=57396|WeeCodes] (making sure you have the "Process WeeCodes" box checked (in the list of checkboxes on the New Comment page).
For example, typing:
[post://199328|Scott's post]
would show up as [link|http://z.iwethey.org/forums/render/content/show?contentid=199328|Scott's post] and be a clickable link to the 13 Things post in the Science forum.
You can get the post number by looking at the http address when you're reading the post you want to cite.
2) Use HTML to create the link yourself. You'd type the above link as:
[a href="http://z.iwethey.org/forums/render/content/show?contentid=199328"]Scott's post[/a]
where you would replace the open and close square brackets, [ ], with open and close angle brackets < >, respectively. Replace the stuff in the quotes to (almost) any other URL you might want to use, and "Scott's post" with whatever text you want to use to describe the link.
The WeeCodes are quite handy. You probably want to experiment with them until you feel more comfortable.
Experiment with the Test forum and post a question if this isn't clear.
HTH.
[edit:] Tyop in Subject. Sheesh.
Cheers,
Scott.