....How do I add 1 point to a finished x spline?

Seems like the simplest thing to do. I just need to add 1 point in an area of a finished x-spline tracing. But for the love of all things it wont let me

Are you using the add points to spline tool and it won’t let you or are you unsure of where that tool is? Please see the link below for the basic toolbar, where you will find the add point tool.
https://borisfx.com/support/documentation/mocha/8.0.0/#_basic_toolbar

I found it. i. the letter “i” works even though none of the buttons on the menu correspond to it

Ah, try moving to classic workspace mode. I think I know what the problem is. Sounds like you are in the Mocha Basics workspace.