Yes, you’ve done step 1 correctly, which is assigning the right interface for Cubase. Step 2 is to assign these inputs to Cubase buses, which is done in the VST Connections dialog (from Devices menu). There are tabs there for input and output: simply create input and output buses and assign your interface channels to Cubase buses. After that, new tracks can be assigned an input and output bus to use for recording / playback.