Is is better to include a hard drive letter as well.
c:\server\www
I tested it without that and it works. It just did the same as when I included "c:". But I don't know if there is a deterministic behaviour in it. Try to look for unziped folder in the root of the hard drive partition you have stored your project on.
EDIT:
I consulted with the DotNetZip spec and they mention that you can provide just a relative path.
Extracts all of the items in the zip archive, to the specified path in the filesystem. The path can be relative or fully-qualified.
But they do not mention where will appear unpacked files if you do. My guess is in the first part of the answer. I have my testing project stored on C:/Users/... and unpacked files appeared directly in the root of C:.