the manual says
Download the plugin, place it on the extensions folder on ocsreports and unzip it.
Go on ocsreports in extension tab. Select the plugin and click on install
Have you done this?
i juist tried it - i copied the full unziped folder with any files to the extension folder on my test debian to
After that i could install the plugin with the extension manager in OCS-Invenotry webinterface-
The uptime Plugin was listet there after copying the folder