A properly installed module may span multiple directories, possibly including compiled libraries, 'auto' directories housing autoload methods in some format, and so on. For this reason it's often not effective to just copy a module to your hard drive and plop it into a directory under ~/site/lib.

There is a good discussion on installing modules, including links to more thorough descriptions, in Catagorized Questions and Answers, here: Installing modules. Read and follow the advice you find there, and your problem will probably resolve itself.