Since Citrix XenServer is Linux, you just have to access by standard linux method: ssh
simpy initiate a ssh session with root account:
ssh root@yourserver.ip
and you are inside.
You will see a welcome text with the instructions:
Type “xsconsole” for access to the management console.
And once you type xsconsole you will get this:

I want to try a machine I got as a vmware image, so I went to install the Free (as in beer) VMware player, this ois what it takes:
For the SAP project, in order to host the expensive Windows Server 2008, Microsoft-SQL and the even more expensive SAP server, the SAP integrators have recommended us to run it in top of Citrix XenServer, in order to have an easier maintenance, migration, backup, scenarios…
Here comes the fun, Citrix XenServer, is no other thing that a Linux Distribution Focused on Xen Virtualization, so in the end, the whole expensive proprietary software is going to end up running in top of Linux (what he knows, we all benefit from).
The bad side, in order to control the XenServer you need a client named XenCenter that only runs in Windows, nevertheless I have read about a coming web version. And about the OpenXenCenter project that will bring a Linux client. (there is of course the CLI option).
Since I don’t use Windows on my laptop I have installed the XenCenter inside the Windows 2008 that is running inside the XenServer, and I can control via RDP by tsclient from my Ubuntu Laptop.