When you ask TeX to use a certain class or package, it usually looks for the necessary files in so-called texmf trees. These trees are simply huge directories that contain these files. By default, TeX looks for files in three different texmf trees:

• The root texmf tree, which is usually located at /usr/share/, c:\texmf\, or c:\Program Files\TeXLive\texmf\.
• The local texmf tree, which is usually located at /usr/local/share/, c:\localtexmf\, or c:\Program Files\TeXLive\texmf-local\.
• Your personal texmf tree, which is located in your home directory.

Finally, you need to rebuild TeX's filename database. This done by running the command texhash or mktexlsr (they are the same). In MikTeX, there is a menu option to do this.