Nelson,
The latest version of Codewarrior Classic for the S12(X) family is v5.1 which supports the entire S12XE family. The "About Freescale Codewarrior IDE" shows the IDE version number of 5.9.0. But the release version (v5.1) is shown at the top of the window revealed by clicking the "Installed Products" button. This will also show any "Service Packs" that have been installed.
The S12XET512 is the same die as the S12XEQ512 so selecting the EQ512 should work. Can you provide any additional details on the problems you're having?
Regards,
Gordon