When you go into the Admin section of Marketo and click on Sales Insight you see two dashes (--) instead of an actual version number.
If the plug in did not install correctly, you will see this in the admin section of Marketo for Sales Insight. You will know if the installation worked properly if you see a version number instead of dashes.
To fix this, install MSI again. You can find the instructions on how to do the installation right here.