THE PLUG-IN SHOULD NOT BE INSIDE OF WP-INCLUDES, IT SHOULD BE INSIDE WP-CONTENT/PLUG-INS
Uninstall the plug-in, if you use a ftp to upload your files, open it up and make sure everything was deleted. Go to add plug-ins. Instead of doing the auto install, choose download. Unzip the file on your computer and put it in wp-content/plugins folder. *** When you unzip the tinymce make sure you open the folder once unzipped. Sometimes it will have a folder level like this:
tinymce.exe (original download)
unzipped example: tinymce, tinymce
The tinymce folder sometimes is two levels deep select the last folder named tinymce and upload this one. I don't know why some plug-ins are like this, but if you they are and you don't follow the steps when you upload it to you ftp site it won't show up as a plug-in.
Go to your plug-ins in the Administrator panel then activate the plug-in. That should do it