How can I use eryx on pc?

I will first answer the question above then explain how to switch other kinds of weapon (demon, angel, guns).

To switch to Eryx scroll down ONCE on you mouse scroll wheel. You can see on the bottom left corner which weapon is equipped.

First of all the default controls are defined assuming you have a scroll wheel mouse, also assuming that the scroll wheel can click. Ex. Logitech MX518.

DmC Devil May Cry has 8 different weapons available to play, divided into the following categories:

Demonic: Rebellion, Arbiter and Eryx;
Angelic: Osiris and Aquila;
Guns/Long Range: Ebony & Ivory, Revenant and Kablooey.
And this is how you equip them:
Demonic: Mouse wheel scroll DOWN
Angelic: Mouse wheel scroll UP
Guns/Long Range: Mouse wheel CLICK