The virtual Boxcryptor volume/disk gives you access to your encrypted files and also allows you to encrypt files. To do so, simply open the Boxcryptor disk in Finder, open the Dropbox folder within the disk. You can then right-click any subfolder and choose "Services -> Encrypt with Boxcryptor". Encrypted files are marked with a green label in the Boxcryptor disk.
Then, install our Boxcryptor app on your iPhone, log in with your Boxcryptor credentials, connect it with your Dropbox, browse to the encrypted file and open it.
You can also have a look at the Boxcryptor for Mac OS X manual to learn more: