If you didn't "Grant access to all users” for “Choose security level” during the install of Marketo Sales Insight please follow the steps below.
1. Grant access to all visualforce pages in the package to all required user profiles.
2. Login to Salesforce
3. Navigate to setup->develop->pages.
4. Click on security next to the required visualforce page that have mkto_si
Your users should now be able to access Marketo Sales Insight.