Download package then unzip, you will have a component and a plugin.
You can install component as normal.
How to install JMS Multi Images Upload for Virtuemart System Plugin: You can read here
How to use JMS Multi Images Upload for Virtuemart System Plugin:
For a new product, you have to save it before use plugin
Edit a product, then go to Product images tab, you will see "Select files" button at the bottom. Click on bottom, then choose images from your desktop, plugin will automatically upload those images, create thumbnails for them and update those images for product by ajax.