The inventory screen for sub-blocks can be navigated to in the same way it can for the main-block, but sub-blocks can also have its inventory accessed from the sub-block profile, in the "Inventory" tab.
To view a room type at an event hotel with inventory already created, you can click on it in the list at the bottom. To create new inventory, select the event hotel and room type from the drop downs, and a "Create Inventory for Block" button will appear.