I guess steam is looking for the library in a different place than the package installs it.
http://askubuntu.com/questions/602637/you-are-missing-the-following-32-bit-libraries-and-steam-may-not-run-libc-so-6
Try creating a symbolic link that points to the library that the package installed. An example command :
ln -s /lib/i386-linux-gnu/libc.so.6 /lib/libc.so.6
You may need to change the part “/lib/i386-linux-gnu/libc.so.6” depending on where the library is actually installed by the package.