If you use Joomla 1.5 so you need to do these works:
1. You need install VirtueMart (in the demo we use VM 1.1.7)
2.You also need to install 2 modules come with the package (Joomla 1.5 version), one for header slideshow and one for product slideshow.
3. If you want it looks very very like our demos, extract the demos package and copy /extracted_demos_package/components/com_virtuemart to your_site/components/com_virtuemart and copy /extracted_demos_package/administrator/components/com_virtuemart to your_site/administrator/components/com_virtuemart.
After that, remember to change the site url in your_site/administrator/components/com_virtuemart/virtuemart.cfg.php.
I have a suggest that you should install the demos package and you will have a conpleted site as demos, then you can follow the configurations in back-end to build yours new one.
There is someone love to use our demos package then modify latter.
If you use the version for Joomla 2.5, it is created for VM 2.0 and you do not need to do the step 3 above. Just install VM 2.0 from VM official site and our modules provided in package.
Thanks for using our products!