Its been a while since I do not upload to microstock and I´ve just found the Mac version of microstocksubmitter but I miss some agencies like Alamy, scanstock or stockfresh where I used to upload content. Perhaps I used the custom ftp service for them but I do not find it in the mac version. Is not enabled?
Also where do you switch between free and paid plans?

Yes Mac OS X version is currently lacking many agencies support compared to Windows version, but we're working on it.
Mac OS X version is currently free to use and unlimited.
For the Windows version you can purchase a subscription using Account -> Buy now menu.