Folks have been waiting a while to get Equation Editor into OneNote (Win10 version... it's been in 2016/Desktop for forever). Now, the funny thing is this won't make a huge deal for me. I tend to just write my equations out, and if it's for more serious distribution I tend to write it in Word. But for others, this may improve the way they work in OneNote. And I also think I'm not allowed to call it Equation Editor, but I'm going to ignore that. Make sure you've updated your OneNote (go to the Store and check for any Downloads & Updates). I recommend folks visit the Store regularly to get any updates. I'm never sure how often it looks for updates on its own and Microsoft has moved to a continual, if gradual update process for all of its apps. To start entering equations, click on the INSERT ribbon and then on EQUATION. You may think, "why not just click on the Math button?" but that is to translate digital ink or text writing into a mat...
...not something ready-made, but something in continuous formation through choice of action. J. Dewey