Dropbox install // Key Missing

Essentially, you need to import the key to your user’s keyring (see https://wiki.archlinux.org/index.php/Arch_User_Repository#Acquire_a_PGP_public_key_if_needed and [FAQ] Issues with “signature is marginal trust”, "signature is unknown trust", or “invalid or corrupted package”)

The AUR is user-submitted content so it’s up to you whether you trust the developer’s key.