In 2.0.x we need to put an extra folder named "wysiwyg" inside the class folder.
In the file you wish to use the koivi editor, you state the following line where the other main includes are stated:
include XOOPS_ROOT_PATH . "/class/wysiwyg/formwysiwygtextarea.php";
If you would like to use only a few buttons you can insert a line like:
$options= array
(
"fontname","fontsize","formatblock","forecolor","hilitecolor","newline",
"undo","redo","separator","cut","copy","paste","separator","bold",
"italic","underline","strikethrough","removeformat","separator","justifyleft",
"justifycenter","justifyright","justifyfull","separator","insertorderedlist",
"insertunorderedlist","indent","outdent","newline","superscript","subscript",
"separator","quote","code","separator","createlink","unlink","separator",
"inserthorizontalrule","createtable","imagemanager","insertimage","imageproperties"
);
Then to use the editor in a form ($form) insert these lines:
$form->addElement(new XoopsFormWysiwygTextArea(_MD_DESCRIPTION, "fieldname", $initial_value, '100%', '400px',$options) );
I do not know where the files you need can be found at the moment. If you like I can send you a zip file with the wysiwyg folder.