The free version of Modelsim is a 32-bit binary and therefore requires certain 32-bit libraries in order to work correctly. For Ubunutu, install the following packages
sudo dpkg --add-architecture i386
sudo apt-get update
sudo apt-get install libc6:i386 libncurses5:i386 libstdc++6:i386 lib32ncurses6 libxft2 libxft2:i386 libxext6 libxext6:i386