Joomlart doesn't show in Backend after Installation

We have been trying to install the MegaMall theme on Magento ver. 1.9.0.1
After copying the megamall theme files, the base theme files and all the extension files to their respective directories defining jm_megalmall as the default
theme in configuration->design nothing happens.
There is no Joomlart tap in the magento back end to be found.
Flushing the cache results in a blank page front end and a -- except for the very top bar -- blank back end.
Alternatively the magento error screen shows up.
At this stage one can only reinstall the back up file.
I have repeated this process now 7 times to make sure that nothing went wrong with the file transfers.
Each time the same thing occurs.
I need somebody to look into the issue as we have been waiting already for several days to get this sorted.