You can type abs to represent absolute value.
y = |x| is the same as y = abs(x)
Another option is to press the math template button (next to the 9), and one of the templates looks like 
 in the upper right corner. Or that symbol is in the lower left corner depending on how your calculator is set up.
 in the upper right corner. Or that symbol is in the lower left corner depending on how your calculator is set up.