You could also try to unzip it with php. Doesn't work on any host, but it's worth a try.
Create a unzip.php file with the following content:
<?php
system("unzip modx.zip");
?>
Upload unzip.php and modx.zip to your webspace and open yourdomain.com/unzip.php in your browser. You should get a lot of messages about unzipped files.
If nothing happens, then "system()" is most likely forbidden on your host.