To import an OpenPGP key pair or a public key, go to your account Settings -> Messages -> Encryption:
For Importing a Key pair/Personal key:
For Importing a Public key:
Moreover, you can export OpenPGP keypairs and other public keys.
If you still are not able to import an OpenPGP key pair or other public key, please send us an email to firstname.lastname@example.org (PGP key).