from what i can see -get is used ONLY to grab a single file. And your format is incorrect, seems the -get needs the full ftp:// address not just a site name. Both of the following work for me.
c:\program files\_internet_\flashfxp\flashfxp.exe -get -localpath="D:\" -remotepath="/My21st/Untitled-1.jpg"
ftp://un:mypass@host:21/
c:\program files\_internet_\flashfxp\flashfxp.exe -get -localpath="D:\"
ftp://un:mypass@host:21/My21st/Untitled-1.jpg
If you wish to transfer a folder which it seems is what your trying to do, i think you have to make a queue file and use -go queuefile.fqf but i could be wrong i dont do a lot with the command line for ffxp.