Sorry for the "noob" question, but I'm missing something about it. I see the crossbow in the selection wheel, but how do I activate the weapon? And when it is on Geralt's back, how do I get him to equip it? I think I got that the mouse wheel click fires the weapon, right?
Grand confusion :(

You press TAB & select it, & use the middle mouse button to fire it.

You must select the crossbow in the selection wheel with left mouse button or E.
To use, you need the 'use quick access item' button, by default that's the middle mouse button aka mouse wheel click. It's the same for bombs. To aim manually you must hold the button, and you can abort your shot using spacebar. Gerald refuses to fire at civiians so practice when nobody is in sight.

Do you have problems using bombs as well?

I think I've gone through the steps carefully. My conclusion is that the mouse wheel click is not accomplishing anything on my system. I've got to go prospecting online with Logitech. I don't expect you to have a solution for a hardware problem like this, but I do thank you for clarifying the procedure This is copied from my reply to From_Dutch, but I wanted to thank you for your help.
I think I've gone through the steps carefully. My conclusion is that the mouse wheel click is not accomplishing anything on my system. I've got to go prospecting online with Logitech. I don't expect you to have a solution for a hardware problem like this.