How do I download folders through FTP in Terminal?
mget is the closest you can get with the included FTP binary. You have to mkdir and cd/lcd and then get all the files than match a pattern like *

The prompt toggle may come in handy. 

FTP works within a directory, not on directories as containers of files. If you absolutely had to get the job done and ftp was the only tool you could use - you could cobble together an expect script to drive ftp for you. 

Thankfully there are alternatives like scp|rsync|wget to get whole directories worth of files.