As XenServer 6.5 is build on CentOS 5.0 we have to rely on Dell to support this OS for OMSA.
As it turns out, they did drop support for CentOS 5.0 in Q1 2015. So instead of adding the latest version of the Dell repo, we have to need the last supported version, which is 2014_Q4.
So, to add the repo use this command:
The rest is exactly the same as in my previous posts.wget -q -O - http://linux.dell.com/repo/hardware/2014_Q4/bootstrap.cgi | bash