Hi Akim
I do have a development platform with Joomla 2.5.4 installed and jOpenSim works fine there with me. Even this website here is on 2.5.4 ...
Did you install successfully other extensions in your Joomla? If the extension manager fails to upload the zip file, this many times indicates missing write permissions on some folders for Joomla.
Greetz
FoTo50