There are a few things you must do to set this up.

First, make sure in Devices - Device Setup - VST Multitrack that the specific ASIO driver for your audio card is selected.

Second, go to Devices - VST Inputs and turn on all desired inputs. Here you can label them if you desire - these labels are the actual input names that show up in your VST Mixer.

Then, go to Devices - Track Mixer and just above the fader you will see IN 1, IN 2, IN 1, IN 2, etc. This is because the default setting was only with 2 inputs enabled. Simply click on these, and choose the inputs for your tracks.

You will probably want to save this as c:\program files\steinberg\cubaseSX\default.cpr so you can start with this project when you launch CubaseSL/SX. To set it to open this at startup, go to File - Preferences - User Interface, and change On Startup to Open Default Project.