I am assuming powdered corn is cornstarch.
If it is, and you're doing all grain, add the cornstarch in with crushed base malt and water during the mashing process. This will help convert the starch into fermentable sugars.
If you're doing extracts, the process is similar if you want to convert the cornstarch into sugars. Steep with a crushed base malt like Pilsner, Vienna, Munich 10L, pale ale, or malted wheat. If you don't have base malts, you can find amylase powder off the shelf at the brew shop. Just add the correct amount of amylase powder in water with cornstarch. If you have an iodine conversion test kit, use it to verify conversion of your mix, then mix with extract or wort.
I do all grain, so any adjunct grains or starches (like corn) will go in the main mash with everything else.