packer build command not building vm autoamtically

[rajeshbavan79]

Hi,
I have changed the ubuntu1204-desktop.json to install ubuntu 12.0.4.4 desktop iso from local box. However, I noticed "headless" build is not working as expected. I am trying to build the vm only for virtual box.
Addition to that, I have fixed the preseed.cfg file to be looked from my local server
"preseed/url=http://localhost:8000/preseed-desktop.cfg ",

I issued the following command

packer build ubuntu1204-desktop.json

and I am getting error about accessing the preseed-desktop.cfg file . It look like vm is not able to get access to the above file ....
Could you please advise ?