I have found that pressing with a steam iron usually works well to remove creases. You can also try submerging the aida cloth in water and allowing to soak for awhile. Then roll in a towel to remove water (wringing will only produce more wrinkles). Use a damp press cloth and press with a steam iron.
In general, the crease will come out on its own, especially once the cloth is under tension for stitching. If you need it out faster, wet the cloth (in cool water) and lay it out flat to dry. You could also hang it to dry. You shouldn't need to iron it, but if you want to iron from the back.