I would completely remove TinyMCE, plugin, files and all (I believe that older versions of MODx also had a module), then re-install it. It would probably be easiest for you to upload the assets/plugins/tinymce folder and the original install folder (download MODx 1.0.3 again if necessary), then run the installer in upgrade mode, clearing all checkboxes except the one for TinyMCE when you get to the install optional items page.
To manually install it (after uploading the assets/plugins/tinymce folder) create a new plugin, paste in the code in the tinymce.tpl file (in assets/plugins/tinymce).
In the Configuration tab, put this in the Plugin Configuration field:
&customparams=Custom Parameters;textarea; &mce_formats=Block Formats;text;p,h2,h3,h4,h5,h6,div,blockquote,code,pre &entity_encoding=Entity Encoding;list;named,numeric,raw;named &entities=Entities;text; &mce_path_options=Path Options;list;rootrelative,docrelative,fullpathurl;docrelative &mce_resizing=Advanced Resizing;list;true,false;true &disabledButtons=Disabled Buttons;text; &link_list=Link List;list;enabled,disabled;enabled &webtheme=Web Theme;list;simple,editor,creative,custom;simple &webPlugins=Web Plugins;text;style,advimage,advlink,searchreplace,contextmenu,paste,fullscreen,nonbreaking,xhtmlxtras,visualchars,media &webButtons1=Web Buttons 1;text;undo,redo,selectall,|,pastetext,pasteword,|,search,replace,|,nonbreaking,hr,charmap,|,image,link,unlink,anchor,media,|,cleanup,removeformat,|,fullscreen,code,help &webButtons2=Web Buttons 2;text;bold,italic,underline,strikethrough,sub,sup,|,|,blockquote,bullist,numlist,outdent,indent,|,justifyleft,justifycenter,justifyright,justifyfull,|,styleselect,formatselect,|,styleprops &webButtons3=Web Buttons 3;text; &webButtons4=Web Buttons 4;text; &webAlign=Web Toolbar Alignment;list;ltr,rtl;ltr &width=Width;text;100% &height=Height;text;400
In the System Events tab, check these events:
OnRichTextEditorRegister,OnRichTextEditorInit,OnInterfaceSettingsRender