If your interface has midi in/out then mapping is possible you just need to figure out how to do it.
If you look under windows->midi maps and add a new command, one of the boxes will tell you whats available to control for each channel. Not everything is available, at least in 3.0. But you can get creative with what controls what.
Here is a guide to setting up a fader command:
1. Open the midi-map window under the window menu at the top of the screen.
2. At the bottom of the window click "NEW".
MIDI Status=Controller.
The next 2 boxes should be "0" & "1".
Route to:=the channel you want to put the midi command on. ex: ch1 FX1 ch25 ect.....
Parameter= Fader
Mode=N/A
Direction=Both
3. In Cubase you need to create a Generic Device by going to the Devices drop down menu and selecting Device Setup.
4. Click on the "Remote Devices" folder to select it, then click the + symbol in the top left corner. A drop down menu will appear. Select "Generic Remote".
5. Make sure that your midi in and out are selected in the drop down menu's at the top of the window.
6. Cubase automatically sets up faders, pans and sends. To start I would delete everything except the first command. You can add the rest back in later when you get the hang of the first one.
7. Now in Cubase 5 it has a Learn function. Basically select the target command, move the fader on the d8b that youve selected to map and press and hold the Learn button while you move the fader a couple of times. Make sure that under the "Flags" column that receive and transmit are selected. After that hit your apply button and that fader should be mapped.
8. Now in case you don't have a learn function then you need to look back at your map window on the d8b and make sure that your Channel and address numbers match up. Once they do make sure your Flag's column is correct and hit apply.
I myself use the FX bank to control 24 faders in cubase 5, this is not a perfect solution because resolution between the 2 sucks. But I only use them to control the output level to the d8b if a track is too hot coming in to the Tape In bank. I've also mapped the assign buttons on that bank to be my rec enable buttons, because rec/rdy button is not available to be mapped on the d8b. There are also a few other things that I've mapped that help out on work flow and keep me from grabbing the mouse all the time.
Hopefully this makes sence and will get you started.
-Casey