Fldigi is designed on Linux and works just fine with a USB-Serial adapter. The most common cause of this problem is that many distros do not include the user in the DIALOUT group, which is owned by root. It isn't a Fldigi problem, it is a system security "feature".
The solution is simple: add yourself to the DIALOUT group, log out and then log back in, and you should be good to go.
I'm not sure exactly how to do that on Fedora 17, but a click google search shows this: http://docs.fedoraproject.org/en-US/Fedora/17/html/FreeIPA_Guide/user-groups.html
which should get you started.
Hope this helps.