At this time, all of our DEB packages are marked as 32-bit (i386). This is because of Ubuntu Software Center limitation that requires .deb files to be marked with specific architecture. If you're having trouble installing DEBs on your 64-bit computer, make sure to run the following Terminal command prior to starting your installation (enter your user password to unlock root priviledges):
sudo dpkg --add-architecture i386
For more information about this issue, take a look at the Some games come with a 32-bit binary only. What does this mean? support article.