Hi Torrey,
All the information you need is right in this Liberry link:
http://www.johnbridge.com/vbulletin/...ad.php?t=40603
Ideally the flange should rise 1/4" to 3/8" above the tile surface. Those extenders can work for you. Make sure the current flange top is clean, and use silicone between it and the plastic extenders. Heed the instructions that come with the extenders, of course.
When you shop at the big box stores, don't ask any questions. Just smile and nod at the help, get what you need, pay, and leave.