Hi,
Open
VM Configuration > Checkout
and enable: Display modal popup upon 'Add to cart'
Attachment Pasted_Image_9_3_13_12_29_AM.png not found
If you have already done it, but still don't see the "modal popup".
Then open:
VM Configuration > Templates and make sure
Use Fancybox is disabled :
Attachment Pasted_Image_9_3_13_12_32_AM.png not found
If still no luck, then it means your website has a JQuery conflict issue which prevents modal popup loading.
Please provide me your website URL and let me investigate your website source codes.
Cheers,
Support & Development