Try these different steps to solve your issue:
1) First, check to see if the editor plugin is enabled:
Tools -> Plugin Manager. Locate your Editor and ensure it's enabled.
Also check Global Config and make sure default editor is set.
You could also try disabling the package you installed and see if it works afterwards. If not try to disable all the editors only leaving the default of Joomla and check again.
2) Change back to one of Joomla's installed templates and see if the problem remains.
If not, then it is a problem with your new template. There is a possibility of javascript problems.
Sometimes enabling the system plugin 'System - Mootools Upgrade' will help.
Otherwise check with your template provider for possible help.
3) Also turn php error reporting on as it may be this problem:
http://whatisgon.wordpress.com/2012/05/04/fixing-call-to-undefined-method-stdclassondisplay-error-in-joomla/
4) Lastly, I like to replace Joomla!'s editor with the JCE editor which has solved many issues like these before.
http://www.joomlacontenteditor.net/