It should have tool tips & recipes in NEI, but basically you connect it to low voltage and put two items in on the left and it makes them into stuff and puts it on the right. Some recipes take a LONG time to do, so you may not immediately notice progress.
Its required for some of the parts you have to make. Regular circuits you can make by hand but there are somethings you have to use that machine now. Just like you need a bending machine to make plates to make mixed metal ingots.