If you are using Linux Mint Gnome or Linux Mint Debian Edition, you will notice that Ctrl+G or Mark All Upgrades is disabled. This has been done to prevent users from installing upgrades that have not been tested and verified to work by Linux Mint team. Read more in this discussion.
Linux Mint Debian Edition is a rolling release distro based on Debian Testing. It is currently only available in 32bit and not for other architectures.