How old is your version of Mach3? In old (really old) versions, you can find what you're looking for in Config > State.
However, adding G91.1 to your g-code is a much better choice than just changing it in Mach3. As the IJ mode is modal, if you load any g-code that has a G90.1, it'll reset the IJ mode to absolute.
Having G91.1 in your g-code will insure that Mach3 runs the code correctly regardless of the state of the IJ mode when you load the g-code.
What you should do is edit your post processor to have it automatically add the G91.1.