After you click the Compare button, an introductory screen opens explaining that you need to select the other machine and data against which you want to perform the comparison.

The machine on which you're running the comparison will be referred to as the Local computer.

The other computer we'll call the Remote computer.

A treatise on basic networking is beyond the scope of this help manual.  I will assume you have the machines successfully networked together.

A file share needs to be created on the Remote computer.

If you're using Windows 98® you will need to enable file sharing before you can create a share.  You do that from Network in Control Panel.  Click the File and Print Sharing button, then check the box labeled I want to be able to give others access to my files.  

Once the share has been created on the Remote computer, it's your choice whether you prefer to map a drive or just to navigate to the share.