Power down the controller, pull out any CompactFlash cards, disconnect the battery (under the left cover), and count to ten.
Now the controller's memory is blank and the serial port is set to the default DF1 Full Duplex protocol at 19200 baud, no parity, 1 stop bit, no handshaking, BCC error checking.
Use the Windows Control Panel to determine which COMx port your USB/RS232 converter has been assigned.
In RSLinx Classic, configure a DF1 Full Duplex driver for the COM port of your USB/RS232 converter, with the Device type set for Logix5550/CompactLogix and the serial speed and framing set for the same as the default controller settings above.
The wiring between your USB/RS232 converter and the controller serial port only requires Pins 2, 3, and 5. It's a "null modem" cable, with pins 2 and 3 crossed.
In RSLinx, open an "RSWho" window, check "Autobrowse", and browse to the controller. The controller's part number and icon should appear in the RSWho window, along with the backplane and network card.
You configure the Ethernet daughtercard by right-clicking on it in RSWho and selecting the Module Configuration applet.
If you have followed all these steps and still not met with success, post exactly what results you do get.
RSLogix 5000 version 15 is stable and straightforward but there's no guarantee it matches your CompactLogix, which may have anything between v13 and v16 onboard. If you have the installation CD for RSLogix 5000, it will include ControlFlash and the firmware files and you can step through the procedure for updating your controller.
Where are you located ? There may be a local RA distributor who can give you some pointers as a courtesy.