Certificates and ServerAdmin 10.5.4

Hi, I am really battling here. I have set up a RADIUS server for my wireless clients, and everything is working fine. The only issue I was having is that clients were getting an error regarding the trust on the certificate when trying to log on and the system then doesn't let them on. Simple I thought - my certificate is self-signed. I have purchased a signed certificate and have all of the relevant files - I thought I would just be able to "upgrade" my self-signed certificate using the ServerAdmin tools. No luck. I can't even import the files - ServerAdmin tells me it is not able to do this. After clicking randomly and messing about for a while the certificates tab on the ServerAdmin now tells me that the certificate is signed by the company I purchased from - but when a client attempts to connect and view the certificate it is still unsigned. Has anybody had success doing this that could provide me step by step information - MUCH APPRECIATED!