Try using the UnleashX dash and look at System > Settings > Network and you will be able to change from Static to DHCP there and I think see both Username and Password. I've never been sure what happens in a case like this or if you change those settings and forget one or the other. Maybe also look at the Config.xml.
By default the Xbox is set to an IP address of 192.168.0.1. What I do is use the Static address option connecting direct from my laptop via a cross-over cable.
For whatever reason FTPing to the Xbox is fraught with difficulties. Usually it is the router or firewall settings to blame.
I dislike the usually recommended Filezilla, it disconnect or refuses to connect at the drop of a hat. Not tried FlashFXP so I won't comment on that. I use CoreFTP LE which is an easily found freeware FTP program I find much more reliable, not perfect, but I rarely have trouble with it connecting. It is very much slower than Filezilla though.