Hi. I assume you have alesis multimix 8 usb.
You can read online manual here: http://img3.musiciansfriend.com/dbase/pdf/man/m_630166.pdf
You will find probably solutions on page 18 (Connection instructions for Windows) or 21 (Troubleshooting)
In cubase you may need to change sound card driver:
Devices -> Device setup... -> VST Audiobay