Apple - Built in FTP client (cmd + k) - can't upload files (Mavericks)

The built-in ftp is read only since forever unfortunately. From Mavericks help:

Servers and shared computers that you can connect to

You can connect your Mac to the types of servers and other computers listed here.

...

• FTP servers

With read-only access, you can copy files from the server, but to copy files to the server you may need another FTP app. Choose Apple menu > App Store to find FTP apps available for OS X.

Maybe try the free Cyberduck or (not free) Transmit.

OS X builtin ftp client in Finder is read-only. I suggest you to use Filezilla.

The FTP feature of Finder only has Read-only access.

The possibilites you have :

• The command line ftp from the terminal
• Cyberduck, a Free App also available on the Mac App Store
• Transmit, (my favourite) a 34$ FTP-App which has a nice feature : It can mount ftp servers like you can mount external drives.