If you hit the arrow button next to the burgundy bookmark icon so it points down, you should see a list of the files contained in that chapter. The split button will not be available if there is only one file in the chapter you selected. if there is more than one file contained in the chapter, you want to select that chapter name with the burgundy colored bookmark icon next to it, and the split button should be available to you.
If every name has a little burgundy bookmark icon next to it, they have already been separated into chapters.
If none of the above works, there may be another issue we haven't seen yet. If that is the case, email us at
support@splasm.com and we can get into more details on what is happening on your end.