Now obviously having it installed on the external wasn't your first choice. If you still have that install on the drive you should be able to boot it.

1. Get the UUID for the install partition on your external.
Boot the DVD and go into Disk Utility. You can see your external. Right click your install partition and select information. You should now see a long alphanumeric string Something like: "CC3DCCBD-1CF3-3FEE-8D50-625375FEBF3D". Copy this Unique Identifier to a sheet of paper and shut the computer down.

2. Boot using the UUID
Start your machine up with the DVD and type

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