You need to set up a mount on the dreambox which points to a shared folder on your PC.
First of all, ftp to the box, go to /var/mnt and create a folder called 'movies'.
Use the web interface to set up the mount instead of the remote and menus.
Enter the ip address of your dreambox into your browser.
Go to CONFIG > MOUNT MANAGER
Enter in the details. (see pic)
Make sure the folder on your pc is shared.
Click mount and after about 10 secs you should see a green tick, that's the mount successful.
On the dreambox, press MENU > File Mode > root > var > mnt > movies
You should see the contents of the shared folder on your PC.
Oh...and the dreambox will only play MPEG movies or variations of MPEG. It won't play avi, divx, xvid, etc.
Hope this helps.