There should be a ".packlist" file in that directory, in addition to the "AdminMisc.DLL".

First, please re-verify that you are looking in the correct directory. Those are the only 2 files there. It is easy to omit the "auto" in the dir name, and end up in the wrong place. (I have done that several times)

You can get the Adminmisc-5008.zip file from
ftp://www.roth.net/pub/ntperl/adminmisc/20030714/
Then manually extract and install the DLL at the required location.

I think the problem has to do with the ppm version you are using. I use Activestate Perl 5.8, and installed adminmisc with no problem.