Hi hallatie,
Congrats on the new Mac!
Diablo II for Mac or PC?
The system requirements for either are listed here
http://us.blizzard.com/support/article.xml?locale=en_US&articleId=20767
The game was released when Mac OS X 10.4 was the latest, and when Macs came only with PPC processors. Now they ship with Intel processors, and the apps and OS need to accommodate this. The instructions go differently for PPC and Intel. The app lists "G3 or equivalent" as a requirement for Mac; that was G4 back then, 100 % PPC.
You
may be able to run the app as Rosetta (in Finder, find the application, command-i to get info, and under "General" select "open using Rosetta". Close that window, and now try launching the app). At least it's worth a try. But... as games tend to be graphic and processor intensive, it may or may not run or run smoothly.
If it doesn't run with Rosetta, would you happen to have an XP version? (You could try that with VMware Fusion or Parallels if you have an XP install disc, or try if Crossover allows you to run only the XP app itself, without burdening it with Windows)
Or an older Mac, say a G4...
I also found this, Blizzard's contacts
http://us.blizzard.com/support/index.xml?locale=en_US&gameId=1