You may need to add a reference to that assembly in the script object. Click on the Script object in Object Explorer and click on the References property in the Properties window. From here you can add a reference to that assembly from the GAC.
Make sure the Microsoft.Office.Interop.Outlook dll is in your Studio folder. If you installed 8.0 without Office support it will be in a sub-folder. Find the correct sub-folder based on the latest version of Office installed and copy the dlls from that folder to the root Studio folder.
Copy the entire contents of the Office2010 folder into your Studio folder. Stop and restart Studio. Now add the reference to Microsoft.Office.Interop.Outlook.dll and OpenSpan.Office.dll to the script container and make sure your project also references OpenSpan.Office.dll.