krw, I don't have any idea how the retention enemas are supposed to work with a j-pouch. Did your GI discuss with you any issues you might expect with this? I do know that the enemas come in different sizes, so perhaps you can try a smaller one (like a 1 g enema instead of the 4 g enema).
Another helpful hint that I got from the UC forum is to squeeze all of the air out of the enema bottle before you insert the tip. That way you're just getting enema, no gas. And make sure, if you can, that you have emptied your bowels before using the enema. I use mine just before bed, and then go lay down on my left side for at least 15 minutes to let everything settle down. After that I'm fine.