There are several reasons to get the error message “Class not registered…”:

1. Using Engine object from application compiled for ‘Any CPU’ target platform on 64-bit OS.
2. Using the OutprocLoader object for loading and unloading the Engine object under account that has not permissions to launch the FREngine out of process server.
3. FREngine.dll has not been registered or has been registered incorrectly.
4. Incorrect adding ABBYY FineReader Engine Visual Components to a Visual Studio 2010 project. This is known behavior described here.
5. Runtime installation was not performed correctly.
6. Wrong Interop.FREngine.dll for .NET developer tools.

Respectively:

1. Try to specify explicitly TargetCPU = x86 in project settings. This article contains the information how to change TargetCPU it in the Visual Studio: MSDN.
2. Make sure that you are working under account that has permissions to launch the FREngine out of process server. For more details, please refer to Developer’s Help → API Reference → Engine Loaders → OutprocLoader (in regards to DCOMCNFG).
3. Try to register FREngine.dll using the following command line: regsvr32 /s /n /i:"<path to the Inc folder>" "<path to FREngine.dll>. Please see for details Developer’s Help → API Reference → Advanced Techniques → Programming Aspects → Different Ways to Load Engine (Notes in the “Loading the Engine object by means of COM into a separate process” subsection).
4. You can either use .NET Framework 3.5 for development or install COM Interop assemblies for Visual Components and register FREngine.dll or VisualComponentsX.dll manually. For more information please see the detailed instruction for this situations in Developer’s Help → Guided Tour → Advanced Techniques → Programming Aspects → Using Visual Components in Different Versions of Visual Studio.
5. In order to distribute the application using the ABBYY FineReader Engine library, you need to install the application and the Engine library on the local disk of the workstation. The runtime installation of the Engine library in automatic mode can be performed only from the command line in silent mode. You should run the setup.exe file from the installation CD-ROM with the command line options described in the Engine Help file (Distribution → Installing the Library in Automatic Mode). You can find more details in the Help file → Distribution. The Engine distribution package includes also the System Administrator's Guide. This guide contains complete information about local and network distribution of applications which use the Engine library."
6. Redistribute the Interop.FREngine.dll file for suitable .NET Framework version. You can find .NET wrappers for FineReader Engine type libraries in folder …\ABBYY\SDK\11\FineReader Engine\Inc\.Net interops. These wrappers are generated for Microsoft .NET Framework version 1.1, 2.0, 3.5 and 4.0. The wrappers corresponding to each version are placed in the v1.1, v2.0, v3.5 and v4.0 subfolders, respectively.

In case these recommendations will not help, please send an AInfo report and the sample project using which such behavior can be reproduced to ABBYY Technical Support Team.