Where is Extras ID settings?

[MCMuckle]

Hello,

I have Joomla 1.5.9 and VM 2.0.20b

At the moment I have 2 products.  One is the main product and the other is an optional extra for the main product.  I understand from reading that it is possible to show the optional extra on the flypage of the main product (this is NOT a variant of the main product, it is an optional extra).

I have searched this forum and read several posts in which Extra's ID is mentioned, I have searched the VM documentation site, and have done Google searches.

I am unable to find where I add the optional extra ID in the main product settings.

Please advise

Thanks