I hope I've understood the crux of your question.
Assuming that your 5.1 card is installed correctly and is working properly, you'll first need to assign Cubase's main output bus to your new soundcard. Do this by pressing F4 (to open the VST Connections window). Click the 'outputs' tab.
Click the 'add bus' button and choose '5.1' from the pulldown menu. This will add a new surround 5.1 output bus. You can delete the stereo bus if you wish.
Six outputs will be created in the popup window. Make sure that each output is routed to your soundcard correctly. If not, click on the output in the 'Device Port' column and re-route it. That should take care of the output of Cubase communicating with your soundcard.
Next, you'll need to assign each of your audio tracks to the newly created 5.1 bus. Let's assume you're starting a new Cubase template. Create a new audio track, and when prompted, choose '5.1' from the pulldown menu. Once your tracks have been created, hit F3 to open the mixer. Make sure that the 'Output Routing' fields at the top of each channel are set to '5.1 out' and not 'Stereo Out'.
Once you've recorded a few tracks, you'll need to position them in the 5.1 field. The easiest way is to insert the "Surround Pan" plugin on each of the tracks. You can use your mouse to position the audio anywhere in the 5.1 field. Left to right, front to back, or any combination of these.
There's a lot more to be said, but hopefully this will guide you in the right direction for the time being. Remember, while using Cubase, you can always click 'Help' then choose 'Documentation' and find the online Operation Manual which has an entire chapter devoted to surround recording and mixing.
Good luck! Let us know if you still have problems....
(Chairface- Bigduggieface>> Everywhere a face-face)