The port number that the SRBox is using can be verified via the Windows Device Manager. To open the Device Manager, select the Control Panel from the Windows start menu. From the Control Panel, double-click System to open the System Properties. Then, navigate to the Hardware tab and click the Device Manager button. The Device Manager will open and list the various categories of devices on your computer. Click the "+" next to the Ports category to expand the list of available ports on your machine. The serial port will be listed as "Communications Port (COMX)", where X is the number of the port (i.e. COM1, COM2, etc). This is the number that you will need to specify in the Port property of the SRBox device (i.e. in the ES file).
Please note that E-Prime is capable of communicating with COM ports 1-4. If your serial port is set for a higher number (i.e. COM5, COM6, etc), you will need to change the port number so that it can be accessed by E-Prime. To do this, double-click the serial port in the Device Manager to open the Properties window. Click the Port Settings tab, and then click the Advanced button. You can then select a new COM port from the drop-down menu.