This had me stumped for a bit, so I’ve written it down. If you have a 64 bit version of Ubuntu and want to install a 32-bit package, you simply add :i386 to the end of the package name like this:

$ sudo apt-get install libstdc++6:i386

However, this didn’t initially work for me as apt-get couldn’t find the package:

$ sudo apt-get install libstdc++6:i386
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package libstdc++6
E: Couldn't find any package by regex 'libstdc++6'

It turned out that my installation only had the 64 bit architecture configured which you can tell by running:

$ sudo dpkg --print-architecture
amd64
$ sudo dpkg --print-foreign-architectures

Note that there are no foreign architectures, which is the problem.

The solution is to add the i386 architecture first:

$ sudo dpkg --add-architecture i386
$ sudo dpkg --print-foreign-architectures
i386

That’s better! Now we need to run an update:

$ sudo apt-get update

Don’t forget this update! I did and wondered why I still had the problem…

Now the installation of the 32-bit package works:

$ sudo apt-get install libstdc++6:i386
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following extra packages will be installed:
  gcc-4.8-base:i386 gcc-4.9-base:i386 libc6:i386 libgcc1:i386
Suggested packages:
  glibc-doc:i386 locales:i386
The following NEW packages will be installed:
  gcc-4.8-base:i386 gcc-4.9-base:i386 libc6:i386 libgcc1:i386 libstdc++6:i386
0 upgraded, 5 newly installed, 0 to remove and 1 not upgraded.
Need to get 4,342 kB of archives.
After this operation, 11.3 MB of additional disk space will be used.
Do you want to continue? [Y/n] Y
... lots of lines as the library installs ...

All done!

Source: AKRABAT