Add instructions in the processor NIOS

do you mean custom instructions ?

open your project with the sopc builder

have a look at menue -> tools -> options

change to the IP search path

make shure there is a path listed that contains your custom ip

custom ips will not be visible where normals ips are in your component library on the left side

instead double clic on your nios core

go to last tab Custom Instructions

your custom ip will be available on the left side, highlight it and add it

now you should see it on the right side with its opcode extension

close the nios core window

regenerate the sopc

recompile your fpga

now your design has the custom ip functionality

if you use the ide, just recompile your application at it should now know your custom ip