Check the CD Changer plug to Resler connection. CD changer has three pins while Resler has four. Brown wire (in CD plug) should go to Ground in the Resler interface, Green-Red in CD plug to +12V in Resler and White-Red-Yellow to I-Bus.
Check com port setting in NavCoder. Try all possible ports.
Try different I-Bus software, such as WinBus or IBusAnaliser. Check if they detect I-Bus traffic.
I heard there was an issue about the Resler USB interface, related to the order in which connections were made, but I do not remember how it went. Try connecting first to PC, powering up and then connecting to car. Try the other way round.
Hope it helps.