You cannot (easily) play Diablo 2 on OSX Lion 10.7+, because Lion no longer supports Rosetta. Rosetta is a piece of software through which PowerPC code can be run on an Intel Macs.
Apple made the switch from PowerPC to Intel a couple of years back. Their reasoning was that the performance per watt of power was far superior to PowerPC in the long run.
OSX worked on both PowerPC and Intel up till Leopard. Snow Leopard could only run on Intel-based Macs, but could use Rosetta to run the older PowerPC apps on the newer OS.
Unfortunately for some, Apple has decided to no longer develop 'Rosetta' going forward, so Lion cannot run PowerPC apps.
There a bunch or method to run D2 on Lion (and above). Such as partitioning (Boot Camp, Use 10.6 [Snow Leopard] or later), Wine (Crossover), or virtualization (Virtual Box, VMware etc).