I have a similar problem
i dont get a popup - but my midilink interface (some weird unbranded/untraceable midi lead - worked perfectly in Leopard) isn't recognised in snow leopard Garageband 09 anymore?
i tried plugging it into midi setup, midi setup sees it - i then hit the E-drum trigger... midi setup crashes!
garrrghhh - and Snow Leopard was going so well.....
any ideas anyone?