[Opserty]

I wouldn't recommend using the above method. Its a lot easier and recommended to use the AdminCP.

Anyway back to the topic at hand:

Go to the AdminCP -> Style and Templates -> Style Manager -> Edit Template -> Registration Templates -> register_rules

Find:

<!-- regular forum rules -->
<phrase 1="$vboptions[forumhome].php$session[sessionurl_q]" 2="$vboptions[bbtitle]">$vbphrase[forum_rules_description]</phrase>
<!-- regular forum rules -->

Below that add your custom forum rules. Use HTML paragraph tags around each part to make them look nice

EDIT: Doh!...or you can use Enigma's suggestion