So I bought a cheap non-hydro handbrake on eBay, and it works in Assetto Corsa, and in the LFS options menu (Sliders bar on the right changes when I move the handbrake) it just won't let me select it. It's slider 0, and when I select it, it says axis 5. I have a G27. Thank you all for the help, just want this to work with LFS!

In Options - Controllers screen:

Open Axes/FF Tab.
Click "Handbrake" then select the axis over on the right.
Nearer the top of the screen "Handbrake" select "axis"