During the installation of Respondus 4, you are asked whether you want to install the Equation Editor. If you don't answer affirmatively to this prompt, the Equation Editor won't be installed. Installing or update other software can also potentially corrupt the Equation Editor.
 
The easiest solution is to run the Respondus 4 installation program again and answer "yes" when asked if you want the Equation Editor installed. Note that reinstalling Respondus 4 doesn't require you to enter your activation password again, nor does it affect your Respondus 4 project (exam) files.