Most of the development docs for OSX seems to focus on Objective C which I unfortunately cannot use in Rainlendar.
You will probably want to use Objective-C to control the GUI, unless you find that you don't need to change much of your existing code for the GUI. However, you can continue to use C/C++ for the backend code: see this document
It's great news that you're working on Rainlendar for OS X!
I do not understand why, but for some reason capital-o Objective becomes _object_ive, including in the url I linked to, so you'll need to manually change that for the link to work :/