If your problem is related to downloading data from your freefall logger, read the documentation for your logger first (Neptune/N3(Audio), ProTrack, AltiTrack, FlySight) and refer to the online troubleshooting tips of the manufacturer (Neptune/N3(Audio), ProTrack, AltiTrack, FlySight). Make sure communications is set up properly as described by the manufacturer (Neptune/N3(Audio), ProTrack, AltiTrack, FlySight) bearing in mind that not every device is compatible with every computer (details available here).
Select the virtual COM-Port provided by the adapter/driver as the 'Source' for downloading from the device in Paralog
On some machines, assigning a COM port of 10 or greater to your USB-IR/Serial adapter prevents proper downloads. In this case, change the COM port assigned to your adapter to a port below 10 (Hint: Bluetooth drivers like to grab up to 10 COM ports...).
On Linux you need to manually enter the port used (usually /dev/ttyS0 for serial connections, /dev/ttyUSB0 for USB connections). Make sure you have the proper rights to access hte port, usually you nee to be memebr of the 'dialout' group.
Answers to other frequently asked questions can be found in the FAQ.