I assume you are talking about editing PROTOCOL.INI. While you can do this, you can also use the MPTS configuration tool to do it. This should be in the System Setup folder with a name like "Adapters and Protocols". Using the configure option, you can select your adapter, and edit it's settings. The advantage of doing it this way, is that the dialogs should help you work out what setting is needed. Also, the setting can be different for different adapters. All the ones I've used have different ways of setting the speed and the mode.
If you don't want to use MPTS, look for a NIF file in IBMCOM\MACS for your adapter. This is what is read by MPTS to configure your adapter, and you should be able to find the settings needed from it.