Those instructions are for Ubuntu, not CentOS.
I *think* the ia32 libs are part of the redhat-lsb.i686 package. But it's not like I've never been wrong.
Still, to allow the 64-bit BOINC to run 32-bit applications in x86_64 versions of CentOS (or Fedora),
according to BOINC's wiki, you need:
# yum install compat-libstdc++-296.i686
# yum install compat-libstdc++-33.i686
# yum install compat-libstdc++-296.i686 compat-libstdc++-33.i686
will get them both in one swipe.
The compat-libstdc++-33.x86_64 package should have been pulled in as a dependency when the 64-bit BOINC was installed.
It appears those files exist in the standard CentOS repos; If I recall, BOINC is in the epel repo.
Or, you could
just restrict your CPU/GPU time donations to 64-bit apps (but note that some BOINC Projects don't have *any* 64-bit applications).