I am not sure if I understood well your issue, but I believe it is the step where you must browse to the location of the agent-cert.ssl file (agent certificate).
By default, the agent-cert.ssl file is located on the CSP Management Server in C:\Program Files x86\Symantec\Critical System Protection\Server.
It is required to export it on each server where the agent installation needs to be done.
Is there any specific reason you are installing a 5.2.3 agent on your W2K8 server? I would recommend you to get the latest CSP release, which is 5.2.9 MP3.