How can I downgrade my kernel?
Very carefully, please review the installation material again. Take your time and ask yourself a couple of things while reading. Do they say that the pre-compiled patches which are included are for an Fedora Core kernel or just for the 2.6.21 kernel in general, there is a difference. If they just say a 2.6.21 kernel then you want a vanilla kernel, one that has not been modified by the fine Fedora folks. In this case download;
get the file: lkn_pdf.tar.bz2 to have handy for review/reference.
This is your best bet, as Fedora an others typically retire (read remove) older rpm packages when newer updates become available.
You may have luck here, do make sure if available you get the kernel for your unlisted version of Fedora;