OpenSSL

Command line stuff

Create a .pem file from a server certificate and a key file

You need both files as ASCII text. If you have the files as a binary file (.der) you have to convert it.
First the key file

$ openssl rsa -inform der -in server_key.der -out server_key.pem

and then the certificate

$ openssl x509 -inform der -in server_cert.der -out server_cert.pem

Copy both files to a complete .pem file

$ cat server_cert.pem server_key.pem > server.pem