Plug-in Manager: Editor - TinyMCE
by Pawblo
go to Plugins-->editor Tinymce-->parameters
Make sure all of the buttons are chosen on the left from "Code Cleanup on Startup" to "Compressed Version" and you will be able to insert any html you want.
Code Cleanup on Startup Off On
Code cleanup on save Never Front Only. Always
Do not clean HTML entities. Yes No
Save Warning Off On
Compressed Version Off On
HTH