Cubase 5 and alesis qx49

Hey Guys.

Can anybody help me, I use ana alesis qx49 with cubase studio 5, how do I get all my sliders and Rotary knobs to work with my VST’s etc…
Example - I open my Vanguard VST, but cant use my rotary knobs to use the cut off or when i go to automate pitch shift or modulation wheel, even tho i’ve pressed the ‘write’ key nothing happens.
Is it something i have to do called 'control mapping ’ to assign all my qx49 sliders and knobs to cubase?
Please help.