I have just done this as I needed it for Downloadable User Roles.
I think your problem is that you exported the certficate in some other format. You need to export in base64 encoded format, or the switch will not accept it. Base64 is NOT the deafult option if you use Windows to export the certificate. You have to select it.
However, if you need the certificate for Downloadable User Roles you can just upgrade the switches to 16.08 and they will be able to download/install the CA from Clearpass automaticly, the first time a client validates/connects. I just tested this and it works.
If you go that route, remember to add the clearpass parameter to the radius-server command or the switch will not know that it can download the certificate.
Example:
radius-server host 1.1.1.1 clearpass