Copy the 'Retro AS-1 VST' plug-in from the 'AS-1 Utilities' folder to the 'VstPlugIns' folder of your 'Cubase' folder. Then change the 'Retro AS-1' control panel audio IO to 'Plug-in'.
Selecting from the VST program menus inside Cubase takes a very long time with the default factory install of 1400+ Retro AS-1 programs. We suggest that you leave just the programs you need inside your 'Retro AS-1 Programs' folder of your 'Retro AS-1 2.1 Folder'. For starters, try leaving just the first 128 programs (the 'best of' programs). Note that once you have created the Retro AS-1 VST instance, you can press the VST 'Edit' button to bring up an editor where you can increment and decrement through the programs quickly.