also make sure that you are in the same usb port you downloaded the driver for. ie. if you downloaded the driver to the far left usb, and then plug the cable in the middle, on some laptops it may not work. ensure you are using the same usb poret you installed the driver on. i had this same problem. i just bought an old laptop that still had the DB9 port still on it off of ebay. a few others did as well. (thats where i got the idea)

 

I have seen over and over on the AEM forums that you should never use serial to USB converters. Martin at AMS also strongly advised against it. Buy an older laptop.