I wanted to post a quick sample that I think might be useful. The escenario that I ran into is when you have two Add-ins that complement each other, a UI Add-in and a Business Logic Add-in. Since the user can enable only one of them and disable the other, you might want to check for this and then ask the user to enable both of them to have all the functionality. To check the status of a particular Add-in, you need the IDriverInfo.DriverGuid (since that is unique) that you used when you registered your Add-in.

System.Data.DataView dataView instance.DriverRegistrations.DataView;
// We look for the DriverGuid of our known addin
dataView.RowFilter "DriverGuid = 'DF7ADD4C-BE62-42e5-9F30-44EAD462AC62'";

(dataView.Count == 1)
    IDriverRegistration driverRegistration 
instance.DriverRegistrations.GetByDataRow(dataView[0].Row) as IDriverRegistration;

Console.WriteLine(String.Format("Addin {0} is  registered and is {1}.", driverRegistration.AddinName, driverRegistration.DriverStatus.ToString()));
"Addin not found");