For those having issues with using Terminal (like I was). Here is what I learned.
After opening the terminal window, you are presented with a black background and a command line with a grey box cursor.
The only commands you will need for today are:
"cd" (which will change you directory to whatever you put after it)
and maybe "ls" (which will list the contents of the directory) (it's LS by the way, just in lower case, though I don't think it's case sensitive)
You can navigate to KSP going the long way, doing it step by step, or you can go straight there.
The long way:
cd // (to go to the root directory)
cd applications
cd KSP_osx
cd KSP.app (yes, on macs you can "enter" the directory structure of an app)
cd contents
cd MacOS (case sensitive I think)
(then run the KSP file by telling the terminal program you want to execute the file in this folder by typing, with the added argument to fix the problem:)
./KSP -force-opengl
The quick way:
/applications/KSP_osx/KSP.app/contents/MacOS/KSP -force-opengl
Above a kind sole has written this as a script which you can run to do it for you, though I'm having permission issues with it so it won't work for me.
If you have installed your KSP in a different directory, you will obviously have to navigate to that directory instead.
This fixed my graphics issue, so thank you muchly to everyone who helped me! (especially my brother who told me you could enter the directory structure of an app (the "cd ksp.app" step) which I didn't know and was the one thing that was confusing the kerbal out of me!)