You would need to look for this code inside each theme file. You are most likely to find it inside functions.php, but every theme is coded differently, so it could be somewhere else as well.
There are many tools you can use for searching inside files, such as Notepad++, Dreamweaver, any decent code editor, and even default functionality in Windows 7 (link).
If you are still having trouble locating the code, you can contact your theme vendor through their support system and have them either tell you where to look or contact us directly through our Contact Form or dedicated support forum.
However, I would like to point out that the only thing our page break button is doing in the editor is inserting the
<!--nextpage--> tag inside posts or pages. That comment is later replaced with the proper HTML code by WP's setup_postdata() function, with which our plugin has nothing to do. What this means is that any permanent fix would have to come from your theme vendor, specially because any changes we might instruct you to make would eventually get overwritten during theme updates.