Are you using R2018b? You need to have R2018b installed in order to use Simulink Onramp. You can check which release you're using by typing "ver" at the MATLAB prompt. If you don't have R2018b installed, you can upgrade to the latest release by going to the R2018b download page:
Once R2018b is installed, opening the MLTBX file will automatically install Simulink Onramp. You then launch it by opening Simulink and selecting Simulink Onramp from the Start page.
Link to Simulink Onramp: