Since no one else has replied, I'll take a shot at it.
There are at least 2 ways, but the easiest is to treat your link
to a Cellar post just as you would with a link to an outside web page.
First, go to the post record that you wish to link to.
Click (upper right corner) on that post's number
The command line will then show the URL for that post.
It will look something like this:
http://www.cellar.org/showpost.php?p=863438&postcount=1
COPY that URL into memory.
Next, create or go into your Reply post and use the "Insert link" button
to PASTE the URL into your text... just as you would for any outside web page.
You can also hide the URL in a word... such as
this