Silverlight Ftp Upload
This is an interesting scenario, since it sounds like you are doing this all on the same machine (both client and server). I am responding to the vague original question, and would love additional data to help answer this completely.
FTP uploading itself is not supported by Silverlight, but the concept of uploading a file is, through an HTTP stream of data.
If you want to do an HTTP upload from a client machine to the server: You can use the OpenFileDialog control in Silverlight 2 and 3 to read that data, then use WebClient to upload it.
If you want to do a download from something your web server can send, to the client machine, then you'd need to use Silverlight 3 or newer, which features a SaveFileDialog control.
All Silverlight file dialogs require a user-initiated action for them to work, function, and the user will need to select the file name(s) for security purposes.
Hope this helps!
Assuming your server is ASP.NET based I would use a simple WebClient POST into a .ashx handler. Have the handler write the body of the post to a file in "C:\User\Uploads".
As long as the ftp server is on the same host as the silverlight app you can use sockets and code your own ftp client, its not very hard and there's plenty of examples out there.
In the meantime, name it Silverlight 5, Sockets got an update: with ClientAccessPolicy.xml the port-range got boosted and an in-browser elevated-trust mode is available.
Therefore I've created sharpLightFtp to enable rudimental Ftp.