Boot into configlive... and ftp into your xbox using the ip listed in the bottom left corner and username x3 and password x3....
If you want to use XBMC as your dashboard....
http://manual.xboxmediacenter.de/wakka.php?wakka=XBMCAsDash&v=3la
Here's how i did mine....
1. Rename the xbmc default.xbe to xbmc.xbe...
2. Edit xboxmediacenter.xml and in between the <home></home> tags at the top, put in E:\Apps\XBMC
3. Upload xbmc.xbe and xboxmediacenter.xml to your c:\ on your xbox.
4. Upload all the other xbmc files to E:\Apps\XBMC
5. Edit the configlive settings and put xbmc.xbe at the top of the boot priority.
6. Now, just select reboot to dash.