DSL can be a little flakey when installing to hard drive. I haven't been able to get it to work without quite a bit of hassle. I think the main problem is getting your virtual hard drive set up right beforehand, and then getting grub installed to the mbr. If you think the install has happened ok already, and it's just a bootloader problem, then here is a possible solution. Download this iso, set it as the cdrom image, install grub, unset it as the cdrom image. Install grub. Reboot, and hope.
http://supergrub.forjamari.linex.org/?section=download
Let us know if it worked.
David