Quad core processor utilization in Adobe After Effects 7.1

The computer has a quad core processor but when using After Effects only one of
the cores is being used as viewed in Task Manager > Performance.

When I check Choose Edit > Preferences the Multiprocessing option wasn't
available.
I checked and the plugin for multiprocessing (MThread.aex) was in the plugin
folder.

Nope. Multiprocessing is only available in CS3. The plugin you are seeing is the
multithreading extension, which in geekish computer talk is a different thing
from multiprocessing. ;-) In simple terms, multithreading will fire multiple
sub-processes of a program, whereas multiprocessing will fire a complete
instance of the whole program. Now per se neither procedure is inferior or
superior, but unfortunately AE's multithreading has always been limited to 2
threads. Therefore, at most 2 cores will be used in AE 7 and if that wasn't bad
enough, only very few effects and plugins actually do. So I'm afraid unless you
upgrade to CS3 or use NucleoPro, you are stucck with the insufficient usage. If
you're feeling adventurous, you could try using some tricks and scripts (find
them via AEnhancers.com) that mimic CS3 behavior in AE7 by launching the AE
render engine as a background process, but it may not work as expected and the
benefits may be minor and are strictly limited to rendering, not doing RAM
previews or working in teh program itself.