I don't known anywhere this is accessible via yum, so you'll have to download and install it by hand. Do this first:

yum install compat-libstdc++-33

Then download the RealPlayer10GOLD RPM and install it.

On 32-bit Fedora Core you may need to delete HelixPlayer with

rpm -e HelixPlayer

before the 10GOLD version will work. x86_64 FC5 does not include HelixPlayer.