you installed non-mysql precompiled postfix from repository (which i mentioned before in posts, so i had same problem as you), you need to yum remove it and install mysql precompiled one. Here is the link to doing that, and building a rpm of postfix with mysql enabled:
http://www.swimminginthought.com/?p=41
Dont worry about the warnings, there will be plenty when compiling this rpm
Dont forget to remove your old postfix before installing this one...