I've checked the software and couldn't find any help regarding its usage or its features which means that you have to access the built-in Help section to locate the Interest operation. If nothing can be found there, then go to the installation directory and look for PDF or CHM files. These are usually user guides that you can use to learn about the way the software works.
As far as I know, Pastel Accounting is an old application and many things changed since its release.
You can also use the support page: http://www.sage.com/za/support