Once you identify the correct com port you may still have difficulties. I have purchased USB/serial devices that work and those that don't. More don't than do. I am not sure if I can say this so I apologize in advance if I am breaking a rule, but finding a good one saved me a ton of frustration. I only use the Gearhead Model CA2000. Sometimes I need to use the CD to load the drivers and sometimes I don't.
I have begun putting these at customer sites where they have a PC close to their Magix (especially if it is something like DuVoice that is only used for voicemail.) It seems lot of people have old PC's that are laying around because they need more memory or something and it doesn't take much to run WIN SPM. I connect the usb to serial to 355 to admin port, put a free gmail address on the computer, associate a free logmein account with that PC, and then I can administer the Magix via the internet. I have pointed out to customers that this is good for them as well because once this is set up they can pull SMDR via hyperterminal so they gain an advantage as well.
(Long winded explanation for why I have worked so much with the usb to serial)