Enter in your Xbox's IP address in the box that says "Server or URL".  Keep the port at 21, and enter in xbox for both the username and password (Assuming your dashboard is using all default settings).  Click connect, and you will see a list of seven or so drives on the right hand box.  Congradulations, you are looking at your xbox hard drive.
 
To transfer files from your pc to your xbox, simply find them on your computer in the left, and drag and drop the to the desired location on the right.

Next, open up flash fxp and click the lightning bolt as picture below.  Select "quick connect", and a connection dialog box will display.

Tutorial: Simple Ftp To Xbox Using Flash FXP

I recommend using a crossover cable simply because it cuts out a lot of potential problems that can be caused by faulty setup of the router, and a faulty router itself.
 
A crossover cable is NOT the same as a ethernet cable, and they cannot be used interchangably.  A crossover cable links a client to its host bypassing a router all together.  An ethernet cable does not have this capability.
 
To Ftp to the xbox, you will need a program called FlashFXP that can be downloaded directly here.

Ftp to Xbox Using a Crossover Cable
 
First things first.  Boot up your xbox dashboard and go into the settings.  Make sure they are configured to run with a static ip address of 192.168.0.3, and that the primary DNS is set to the IP address of your computer.  Since we will be setting up your computer's IP address, change the primary DNS to 192.168.0.4.  Hit save before you exit the settings screen, and reboot your xbox.  Check that the correct settings are displayed.

Next, physically connect your computer to your xbox with the crossover cable.  On your computer, go to Start---->Control Panel--->Network and Internet Connections-------->Network Connections.  Right click on your local area connection and select properties.  Select the item that says "Internet protocol TCP/IP, and then click the properties button.  Check the radio button that says "Use the following IP address", and enterin the IP address of 192.168.0.4.  Change the subnet mask 255.255.255.0.  Click Ok.  If you are running any OS earlier than windows xp, your computer will need to be rebooted.

So you've sucessfully softmodded you xbox!  This tutorial will show you how to ftp games to your xbox hard drive.
 
Ftp stands for File Transfer Protocol.  In lamans terms, it is the industry standard protocol for the transfering of files from a host (your pc) to a client (your xbox).  What does this mean for you?  All you emulators, roms, homebrew apps, and of course, Xbox Media Center, can all now be loaded on your xbox following the completionj of this tutorial.
 
You can ftp to your xbox one of two ways.
 
1) Using a crossover cable and link directly your computer and your xbox.  (Recommended)
2) Using a router and standard ethernet cables to indirectly link your pc and xbox.