Where OSX are kernel modules located?

The OSX uses loadable kernel (like BSD and Linux) modules (or kexts) - each one is stored in its own directory (e.g. Dont Steal Mac OS X.kext ) and these are mostly stored in a couple of directories:

/Library/Extensions
/System/Library/Extensions/

Though some others are stored in some other directories (e.g.  /Library/StartupItems/). You can list the currently loaded kernel modules using the kextstat command.

There also some info more info another post of mine on tracking down problem kernel modules in osx.