The initial runlevel isn't normally set by grub, but in a configuration file called /etc/inittab.
Open this file in a text editor (I use emacs):
sudo emacs /etc/inittab
There should be a line in the file that reads:
Change this to read:
and save /etc/inittab (in emacs, press ctrl+x then press ctrl+s, then to exit ctrl+x then ctrl+c).
Once done, you should be able to reboot into runlevel 5.
It is possible to set the runlevel in the grub command-line; edit the boot command that you are interested in and add 5 to the end of the command-line. But that will not normally get saved between reboots.