"My words" is just a list of every word you have studied on Renshuu at least once. It's not really a 'list' so much as a tracker for what you have learned; you can't add words in there to be studied later or use that list in a schedule or anything.
"Studypad" is a pre-made, private list attached to your account that you can fill with any words you want to study.
If you really want to make a schedule out of all of the words you've studied previously in Renshuu, go to "Advanced Search" in the menu, populate it with "My words", and make a schedule out of it. That schedule will not auto-update with any new words you learn though; it will only contain what was in "My words" at the time that you made it.
