You need to edit the grub.cfg file from a live cd (although there's a warning on the file that you shouldn't edit it manually, yet editing only timeout value wouldn't do any harm).
Boot from LiveCD. Go into 'computer' & identify the label of the drive where mint is installed (if it is not already labeled, do it now for convenience). Say it is labeled as "mint", then enter this command in terminal
Look for the line that says timeout=0, and replace 0 with 10.
sudo gedit /media/mint/boot/grub/grub.cfg
Now reboot normally. You should get 10 sec. on the grub menu this time. Boot into mint and follow what garvinrick4 suggested.
And don't forget to runafter editing the /etc/default/grub file.