Adding public key of iOS Prompt app to authorized_keys
For starters, just to clarify the terminology, when you create a key pair, the private key (typically named id_rsa) goes on the client (in this case the Prompt app), and the public key (typically id_rsa.pub) goes in the ~/.ssh/authorized_keys file on the server you're logging in to (in this case your Mac).

You should be able to just drag the private key into Prompt via the iTunes documents interface. If you're getting a connection error, try doing it with your device attached via USB, not Wi-Fi (I've found Wi-Fi syncing can be a bit flaky at times). If that still doesn't work, open up your private key in a text editor, copy the text and send it to yourself in an email (or any other means that gets the text to your device). You can then copy that text, and when you click the key button in Prompt, it should give you the option to Import Clipboard Key.