Safari Download Manager would be 100% pointless to install even if it did work.
In ios 5...you can download almost anything from within Safari without any type of Safari tweak installed.
The only way to utilize Safari's native ability to download files is to have iFile installed.
Once a download finishes you'll just see a blank page with the file in the center of it...just tap "Open in iFile" and iFile opens giving you access to the downloaded file.
Files download from Safari are stores in one of 2 places:
/var/mobile/Documents.
Or
/tmp.