Edit boot menu when booting from a live CD

First answering your specific question,

How do I edit the grub boot menu when booting from a live CD?

I know you want to find a simple solution for your simple question, but I'm afraid that there is no easy solution for you, because almost all live cds are boot with isolinux, i.e., not grub, so there is no way you can edit grub when booting from the live cd.

You can re-create the live cd to boot with grub if you want. Ref: Booting CD with grub

Or your can learn how to use isolinux to put in that nomodeset. It's quite simple.

Now answering the question of "Edit boot menu when booting from a live CD"

All live CDs are using isolinux to boot.

Using isolinux to change boot options (e.g., to put in that nomodeset) is quite simple:

1. press TAB at the live CD boot up screen
2. change boot options (e.g., appended nomodeset options at the kernel prompt)
3. press enter to boot the live cd with the new (nomodeset) option

For about every live cd I've ever used, you can press "tab" (also try "space" or one of the "F" keys) and click "e" to edit the boot options before you fire it up. Yes, this even works with isolinux. Experiment with the timing, but I find it works when the text announcing the bootloader is on screen but before it starts unpacking. Hit "tab" (or other), then wait for a couple of seconds, then hit "e" and edit away.

There is, to my knowledge, no easy way to make this permanent with a cd. It can be done if you know the deeper mysteries of rolling your own Live CD. Or, at least configuration and rerolling thereof. Debian makes it easier but it's still a learning curve for people new to Linux.