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.