We did explain upgrade process few posts ago. Upgrade happens on old version not on the one which you want to install:
https://forum.mikrotik.com/viewtopic.php?f=21&t=129034&p=641456#p641392
Upgrade from 6.42rc20 (not from rc23):
http://forum.mikrotik.com/t/radius-server-not-working-in-2-8-11/127/1
If I did understand post correctly, then this is about fresh installation, not an upgrade:
http://forum.mikrotik.com/t/radius-server-not-working-in-2-8-11/127/1
Version from which upgrade was made is not mentioned:
http://forum.mikrotik.com/t/v6-42rc-release-candidate-is-released/115077/216
I would also like to remind that rc versions are beta versions for RouterOS enthusiasts who are willing to test new features and can Netinstall/re-install RouterOS if necessary.