Many software products, especially the commercial ones, are distributed as 32-bit packages. This means that they won't be installed on your 64-bit system unless you clearly specify that you want to override the architecture dependency.
If you're using Ubuntu or any other Debian based distribution, this post will teach you how to install 32-bit deb packages on your 64-bit OS.
Is it possible to run 32-bit applications on 64-bit OS?
In Unix world, yes: it is quite possible to run 32-bit binaries on 64-bit OS. There should generally be no problem, but there are, as always, a few caveats: