Click Places → Connect to Server....

From Service Type choose Public FTP for an anonymous connection or FTP (With Login) if you have a username.

Enter the server address in Server, for example ftp.ubuntu.com.

Enter Port and Folder if you need to, this is optional.

If you chose FTP (With Login) you will also need to enter a username.

If you want a bookmark, click the checkbox and enter a bookmark name.

Click Connect.

If you chose FTP (With Login) you will be prompted for a password.