With the latest version of CAD Pack, 2012.01, there have been some instances of errors related to an inadequate version of the Microsoft .NET Framework. The error messages can be cryptic, and so it's not obvious that an update to the .NET Framework will fix the error.
Here's one example. The important bit that indicates an error with the .NET Framework is near the bottom. "System.Design, Version=4.0.0.0"
The other error message that presents itself is more explicit, stating that the .NET Framework needs to be updated.
Here is a link to download and install the .NET Framework 4.0 (Full). As we add more features to CAD Pack and attempt to make the application more stable, we leverage new functionality not available in older versions of the .NET Framework. We're sorry for the incovenience. In most cases the Framework on machines running AutoCAD has already been updated, so this error is relatively rare.