OneCalendar can be used from within OneNote as part of Onetastic. If you want to launch it from start menu or the taskbar you can download the standalone version of it.
You can get the latest version of OneCalendar from the Download page. Once you download the zip file, extract it to a folder on your computer. You can extract it to Desktop for instance and launch from there.
Once you download standalone version of OneCalendar, you can right click on it to pin it to Start Menu or Taskbar.
Once you pin it to Start Menu or Taskbar, its icon will be available there and you can launch it from there just like you can from inside OneNote.
If your question isn't answered, send an e-mail to support@getonetastic.com.