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.
Since Linux Mint 9 Isadora is based completely on Ubuntu 10.04 Lucid Lynx, you can add Linux Mint repository to Lucid and make it look and feel like Linux Mint (additionally you may have to do more than just mint specific applications such as changing themes and tweaking panel but you get the idea). You may also like to install a few components that you like in Linux Mint, for example just the mintmenu or mint-flashplugin-64 (if you want a 64bit version of flash for your 64bit version of Ubuntu).