OSX ssh-agent: no password pasting, and problem with PKCS#8?
The dialog for ssh-agent can be circumvented by adding the key in the console/terminal: ssh-add ~/.ssh/id_rsa. You can then paste the password into the terminal. Also, adding the -K option to ssh-add will save it to the keychain as per Oliver Lacans comment.
As mattmcmanus said, id_rsa encryption using PKCS seems to be broken on OSX Mavericks ssh-agent. The fast workaround is to decrypt the keyfile, and encrypt it again with the standard ssh procedure (Key-derivation method: MD5...):




mv id_rsa id_rsa.pkcs
openssl rsa -in id_rsa.pkcs -out id_rsa
# enter passphrase to decrypt
chmod 0600 id_rsa
ssh-keygen -f id_rsa -p
# enter passphrase to encrypt again