I made another of the organizer wallets today from this pattern:
http://www.lbg-studio.com/2011/10/12...organizer.html
I was able to add the elastic pen loop in the center very easily. I tucked the raw edge of the elastic underneath the side of the pocket that holds the notepad and stitched it, then I did another short stitching to make it the correct width for holding an average pen (you can see here that it's holding my fabric marker). Hope that makes sense!