As thcsoft said, you can go grab the ppd file yourself and then have ppm install it. You should be able to find it by going to activestate's site and browsing their repository, and then download the file. Or google(DBI ppd) to find alternative repositories that have it. Or, in ppm, try the command "search DBI" and see if it is able to list anything.

Another good resource for information is the perlmonks tutorial A guide to installing modules for Win32, specifically the Creating a local repository section, which describes how to use ppm to install the package once you've found the ppd file and saved it locally.