PCI-based modems will normally be automatically detected. If your particular modem was not (ISA modems will not be), you need to specify which port the modem is using.

The text on the left side of the buttons display the Linux name for serial ports, and the text on the right side uses the Windows name.

If you're still not sure which port to select, you can find this information from the Windows control panel and configure a dial-up account after the installation is complete.