You haven't explicitly said where the files are now (on an external device, in a folder on your computer, etc.), but the easiest way is to simply drop the files onto the iTunes icon in the Dock.
Note that the importing behavior is controlled by selections in:
iTunes > Preferences > Advanced and choices in "Keep iTunes Media folder organized" and "Copy files to iTunes Media folder when adding to library."
In addition, you should check out the support files at: