If you try to upload and unzip zip files bigger than the PHP memory_limit, it will fail. But with a memory_limit of 8Mo, most of the memory could be taken by PHP itself (depending of the installed PHP modules) leaving little memory left to unzip a file.
But this is probably not an issue for module installation unless you try to install a module weighting several Mo. Your issue is probably due to bad access rights on the module directory. To install modules, the module directory at the root of your Claroline install (NOT the one in claroline/admin !) must have drwxrwxrwx rights to allow Claroline to unzip and install the modules. Please check that the access rights are correctly set.
If this does not work, I'm afraid you have to check the error log to know what happens when you try to install the module.
Frederic Minne (ZeFredz) - Université catholique de Louvain - Contributor to Claroline