Ok, I managed to import most of the demo content except for the images.
It is very strange, when I go to Media and try to upload an image, the upload button doesn’t work.
I have tried disabling plugins and switching themes, and it still doesn’t work.
I believe there is an issue with your installation of WordPress or a conflict.
You can try deleting your site and installing a fresh copy of WordPress and re-installing the theme.
Also, make sure you hosting environment meets the WordPress requirements.