If you would like to have a spell check button for your users to use with their PMs, simply edit your pm_newpm templete.
Find:
Code:
<input type="submit" class="button" value="$vbphrase[preview_message]" accesskey="r" name="preview" tabindex="1" />
Below this add:
Code:
<input type="button" class="button" value="Spell Check" onClick="spellCheck('vbform', '$editorid', '2');" />
Now you and your users can check the spelling in your PM's