I think I can explain this for you, and anybody else who can't figure this out.
Download the Linux version of Cura from the official website, https://ultimaker.com/software/ultimaker-cura
Activate "Linux" or "Linux Apps" in the chromebook settings
Move the newly downloaded Cura Appimage to your linux folder in the built in files app.
Open a linux terminal
Run "chmod a+x {insert file name here}
Run "./{insert file name here}
Like others have said, you will not get a shortcut to open Cura. You will have to type "./{insert file name here} everytime you want to run the app.
One more tip, I recommend you change the file name to something more simple, such as "Cura.AppImage" suggested by another user.
Good Luck!!!