I'm not a big fan of illegal roms so I decided to make a simple tutorial on how to patch the steam version of Sonic 3D Blast:
Prerequisites:
-Sega Mega Drive & Genesis Classics and Sonic 3D Blast owned and installed from Steam.
-Delta Patcher: https://goo.gl/hr48iF
-The Patch file itself: https://goo.gl/NAMXJK
-PscdPack (be sure to download the lastest release): https://goo.gl/MsU3oC
How to:
-(IMPORTANT) Make a backup of your original Sonic 3D Blast ROM, the ROM is located at: " C:\Program Files (x86)\Steam\steamapps\common\Sega Classics\data " under the name: "g0008.pak"
-Open PscdPack.exe and click on "New/Open", navigate to the where the original ROM is located (C:\Program Files (x86)\Steam\steamapps\common\Sega Classics\data). and open the original ROM called "g0008.pak". Click on "Extract ROM" and save it on the Desktop.
-Now open Delta Patcher: on "Original File" be sure to use the ROM we extracted on the Desktop using PscdPack, on "XDelta Patch" use the DX patch file. Click on Patch File. Your Rom should be Patched, and you should be able to play it on any MD or Genesis emulator.
If you want to play it on the Sega Mega Drive & Genesis Classics Launcher:
-Open PscdPack.exe and click on "New/Open", navigate to the where the original ROM is located (C:\Program Files (x86)\Steam\steamapps\common\Sega Classics\data). and open the original ROM called "g0008.pak". Click on "Replace ROM" and select the patched ROM located at the Desktop. Exit PscdPack and be sure to save it.
Now we should be able to play the DX mod legally on steam:
This looks like a great guide, I don't suppose you know how to do the same on the Mac though? I can confirm that at least MultiPatch 1.4 can patch the file properly, the latest version (1.6.2) I haven't tested, it should work although someone else here had problems getting it to. Now all we need is to locate the rom file on Mac Steam, and find an alternative to PscdPack!