Which PLC model are you using? If it is M-series, Nano-10, F-series or FMD then you will need to run the TLServer if you are programming via the serial port. Please refer to the Programmer's Reference guide on starting up the TLServer and testing the serial connection between the PC and PLC, then you can select the "localhost" IP address to connect to the PLC.
Nano-10, FMD and F-series can also be programmed via their Ethernet port which does not require you to run the TLServer.
E10 and H-series PLC are programming using the WinTRiLOGI software and you need to select the PC COM port from the "Setup Serial Port".