I'm glad to hear you have an upgrade plan in place as 5.3 has not been supported for a few years now. I think the latest we support at this point in time is 7.0.
You do not find the agent installer because it did not exist at the time. It came with 6.0 only. Simply download the server installer, run it and select the advanced mode and only select the agent part (comes with the tools as well).
That should take care of your issue.