The "Server interface" field in tftpd32 should be IP address of the interface that is connected to the switch (i.e. an IP address on your workstation that is reachable by the switch). Make sure to verify your firewall is allowing UDP port 69 as well.
In a nutshell, you are starting a service on your workstation that is hosting the local directory in "Current Directory" via the TFTP protocol (UDP port 69), which is native to the Catalyst switch. You need to make sure the image is in the Current Directory, and issue the commands in the link to transfer the image from your workstation to the switch's flash storage.
Once there, you can specify via the configuration register and boot statements which image is to be loaded. If there is not enough space on the switch, transfer the file from the switch to the workstation first (make sure you don't delete it in case the new image does not work).
