The simplest way is as per the notes provided in Downloads:
- add includes/wysiwyg/tinymce.inc before you include header.php
- add class="mceAdvanced" to any textarea tag where you wish to evoke the editor.
Find the file where you can add the include before the header, probably where the other includes are in "modules/ForumsPro/posting.php".
You will know if you have successfully included tinymce.inc because you will see javascript between the <head> </head> tags when you view the page source.
Find the template containing the input tag, type="textarea", probably "themes/your_theme/template/forumspro/posting.html" to add the class - you can view your page source to verify its presence.
You will also need to disable the existing bbcode that is used and change the getlink to allow conversion of the html code.
I use the word probably because I just don't the time to go through the code in-depth - these two areas just seem the most likely. As you have probably already gathered, ForumsPro will not be a simple implementation like most other modules.