Is the binding you are making single-fold or double-fold? Most quilts are bound with single-fold binding, where the binding is simply folded once. This provides a double layer of fabric at the edge, where quilts get the most abuse. I believe the binding machine comes with double-fold attachments; you can buy the single-fold attachments separately. (I have seen them at Walmart.)
Normally you do sew binding twice, even if you are using double-fold binding. It makes for more accurate stitching. A lot of quilters machine sew the binding to the front, then hand sew the binding on the back (for single-fold).
Here is a website that describes the most typical binding method:
http://www.nancysnotions.com/jump.do...age=bindaquilt
I use single-fold binding but I find I get better results if I do *not* iron the binding in half first, so I do not have a need for the Simplicity machine.