You will find the System Monitor application in the Systems Tools menu of the Accessories menu of the Programs menu.
When you choose the Add Item from the Edit menu, a dialog box will be display listing several categories of items on the left and items within those categories on the right. The category list will include File System, Kernel, and Memory Manager. It might also include one or more network related categories, such as Microsoft Network Client and Microsoft Network Server. The Modem category usually does not show up.
For more information see:
Bytes Received and Sent by a Modem