Following instructions very carefully, each time I tried to save after entering paths for .pem files, I ended up with a white screen. To get back in, I had to rename the .pem files on the server.
Here's what was in the DB (_cbsubs_gateway_accounts, params) when I was seeing the white screen:
paypal_encrypted=1
paypal_public_certificate_path=/home/mydomain/etc/paypal_cert.pem
paypal_private_key_path=/home/mydomain/etc/my-prvkey.pem
paypal_public_key_path=/home/mydomain/etc/my-pubcert.pem
paypal_private_key_password=
paypal_certificate_id=EMQVQDM...