In order to send an encrypted PGP message, we need to obtain the recipients public PGP key. There are two common ways to obtain another person's public key, you can download it from a server or obtain a copy of the the key via email/web.
Let's obtain a copy of Bob's key (key ID is 5D24E4AA).
If the recipient sends you the PGP key in an email or you obtain it from a website, it can be easily imported. Let's assume that you have copied and pasted the key into a text file or you have a file with the appropriate key.
Your key manager should display new keys available now.