Admin Control Panel -> Appearance (tab) -> Styles & Templates -> Templates -> register_form
Search and find:
Rich (BB code):
<dl class="ctrlUnit">
<dt><label for="ctrl_email">{xen:phrase email}:</label></dt>
<dd>
<input type="email" name="email" value="{$fields.email}" dir="ltr" class="textCtrl" id="ctrl_email" />
</dd>
</dl>
Below the red code, insert the contents below:
Code:
<p class="explain">Blah blah blah</p>
So the result will be something like this:
Rich (BB code):
<dl class="ctrlUnit">
<dt><label for="ctrl_email">{xen:phrase email}:</label></dt>
<dd>
<input type="email" name="email" value="{$fields.email}" dir="ltr" class="textCtrl" id="ctrl_email" />
<p class="explain">Blah blah blah</p>
</dd>
</dl>
Of course, replace "blah blah blah" with anything you wish,