This looks like a problem in the configuration of your PHP installation. To test this try a whether a normal file upload works (you can find sample code in the php manual if necessary). There might be other restrictions like safedir or something similar in effect.
Claudia aked if a file upload with an other (not ez publish) php file upload script works...
The directory /upload_tmp is a gerneral PHP directory where temporary files are stored. The path is controlled by the php.ini, but you need to have full access to the server to change the path.
As far as I unsterstand, you do not have root access to the server. To make ez work, you will have to concat you provider and report him that problem.
What packages did you try to install? And what error message (if any) did you get when trying to install a package you downloaded before?
Also make sure that the var directory and its subfolders have all necessary permissions, maybe EZ cannot store the retrieved packages because it does not have access rights to these folders.