Looks like "Register & Checkout" and "Guest Checkout" modal popups works just fine. The only problem you have: Your joomla doesn't let visitors to "Create new account". You need to enable it.
- To enable "User Registration"; Open Joomla Admin Panel and navigate to
System > Global Configuration > Users Manager > Component> Allow User Registration. Set it to '
YES' and save changes:
Attachment Pasted_Image_4_13_16__3_10_AM.png not found
- To fix the "scrolling" problem..
Open this CSS file:
/templates/rt_callisto/html/com_virtuemart/assets/css/flexible.css
scroll to the very bottom and include this CSS rule:
div.modal-body { overflow-y:inherit; max-height:inherit;}