You have to use the terminal (Apps>Utilities>Terminal). The best thing to do is type "man ftp". That will give you the manual for the ftp app.
Basically, just type "ftp
http://www.servername.com" and then you will be asked for your log/pass.
If you've never used unix before, navigation can be tricky...
"ls" lists all the files and folders in the current folder,
"cd dirname" will move you to the folder called "dirname",
"cd .." will take you back a directory,
"get filename" will download the file called filename, and
"put filenamepath" will upload the file at whatever path you put into the current directory.
Like I said, it's hard if you've never used Unix before.
My suggestion is "NetFinder". It's Mac-like, so it's easy to use.