They are both on one CD set. Read the manual for instructions for macs.
EDIT: If you don't have the manual, shame on you! ;] I have mine so here are the instructions for Mac:
From the manual; said:
Installation Of Diablo II (Mac)
Place the Diablo II Install Disc into your CD-ROM drive. Double-click on the Diablo II Icon, and then double-click on the "Diablo II Installer" to copy the required game and system files to your hard drive. The installer will present an abbreviated "Read Me." Please read this, as it will contain the most up-to-date information about Diablo II.
When Diablo II is run for the first time, a video test is run to determine the video hardware available on your machine. Your monitor will go blank for several seconds as the program tests the various options available. After the video test is completed, the Startup Options menu displays with the following sections: Video: This section shows the graphics mode in which Dialob II is currently configured to run. To change to another mode, click the "Configure Video" button. From this menu you can switch to other graphic modes your computer supports. Mouse: If you have InputSprocket installed, you can use a multi-button mouse with Diablo II by clicking the checkbox next to "Use Multi-Button Mouse."
A "System Volume" slider is also available to alter the system volume level.