Hi,
Can you provide me your Live URL and let me see it? What do you mean, the window is opened but no form. Are you getting some sort of error code?
to add the same register form in your non-virtuemart related pages, the button uses this HTML:
<a class="FlexibleResgiterButton" title="Register And Checkout" href="/index.php?option=com_virtuemart&view=user&task=editaddresscart&addrtype=BT&tmpl=component&flexible=register" rev="iframe|480|560" rel="facebox">Register And Checkout</a>
But this Button will be styled the way it is if the "FlexibleVM2Gloval.css" is loaded in the page, because this CSS file includes the CSS rule for "FlexibleResgiterButton" which is used in class="". Also, this button gets opened in a modal box which is called "facebox", this facebox is a plugin that comes with Virtuemart, and it is loaded automatically with virtuemart related pages. You should load it manually in other non-virtuemart pages as well. Or use a different plugin to have a similar lightbox effect. (lightbox, shadowbox or something else, there are more than 20 different modal boxes available, facebox is just one of them. You are going use the "iframe")
Cheers.