Please, try this:
<override_macro name="message_control" requires="node_page">
<div class="field-box light-border-color">
<div class="second-font field-title"><t>Message</t></div>
<div class="weak-color">
<n.if.visitor.is_registered>
<then>
<n.html_format_field.checkbox />
<label for="[n.html_format_field.name/]"><t>Message is in HTML Format</t></label><br/>
<div style="margin:.1em 0">
<n.editor_toolbar
textarea_id="[n.message_field.name/]"
original_text="[n.if.page_node.is_post][then.page_node.message_quoted/][/n.if.page_node.is_post]"
/>
</div>
</then>
</n.if.visitor.is_registered>
<n.message_field.textarea wrap="SOFT" tabindex="2" style="min-width:30em;max-width:55em;width:100%;height:20em;" />
<n.new_post_extra_fields/>
</div>
</div>
<script type="text/javascript">
Nabble.quoteOriginal();
</script>
</override_macro>