I didn’t see this anywhere so I thought I would add for others. Here is the process to add a tinymce plugin for Modx Revolution. Please refer to
http://wiki.moxiecode.com/index.php/TinyMCE:Plugins for details on the plugin name (plugin_name) and the button list (button_list) for a particular plugin:
1. Go to System Settings
2. In the drop down where is shows core, click on the down arrow icon and choose tinymce
3. On the "Custom Plugins" setting double click on the value entry to go into edit mode
4. Go to the end of the value and add the desired plugin value as ",plugin_name" (without quotes). For example, to add the layer plugin add ",layer" (wihtout quotes) to the end of the values.
5. Click in an empty area to save
6. Add the button_list to tiny.custom_buttons3 to put the plugin editing buttons in a separate row or add the button_list to tiny.custom_buttons1 or tiny.custom_buttons2 to add to one of the current rows. For example, to add the layer plugin buttons add "insertlayer,moveforward,movebackward,absolute" (without quotes) to the end of the tiny.custom_buttons value list.
7. Click in an empty area to save
You should now see a new button or buttons for the particular plugin.
