So Filezilla doesn't like network drives. That's probably a bit harsh and it's just that windows hides a lot of the mess behind the connection. There is a helpful stickyed post for how to set this up on the official support forum and would be worth following if there is a need to transfer to the mapped drive location. However, if the transfer is coming from the location to a remote FTP site, you will find addressing the network location using the UNC directly will suffice.
Type it into the local site (sic) box and it's job done! =)