When you installed Dreamlinux, did it prompt you with where to install the bootloader (aka grub)? I've never installed Dreamlinux, but I would think it would default to installing its own bootloader to the MBR of the boot disk (e.g. /dev/sda) and (if it was nice), include an entry to boot Ubuntu as well. It could also have offered to install grub to the boot partition instead (e.g., /dev/sda3), in which case, when you rebooted the PC, the Ubuntu grub (which doesn't know about Dreamlinux) would boot the Ubuntu OS.

It should be relatively easy to fix, though, provided that your Dreamlinux installation is otherwise okay. You can boot into Ubuntu, find the correction partition info for Dreamlinux and manually add it to your grub config file (/etc/grub.conf).

BTW, to see the grub menu (Ubuntu's or otherwise), try hitting the [Tab] key after you turn on/reboot the PC, right after it does its POST/memory counting stuff but before it starts booting the OS.