First download WYSIWYG Module and install it from Administer -> Site Building -> Module section.
Now go to the Administer -> Site Configuration -> Wysiwyg
When you click on it, you will see the following screen:
Here you will see a link TinyMCE (Download)
Click on it and download TinyMce editor and keep it the following location:
Extract the archive and copy its contents into a new folder in the following location:
So the actual library can be found at:
Once, you do the above, you will see the another screen look likes:
select tinymce from dropdown for both the dropdown and then click on SAVE button.
and done your editor is now working...