問題：
A reference to the "....dll" could not be added.Please make sure that the file is accessible and that it is a valid assembly or COM component
Normally, if it is a .NET dll, we can use Add reference to import it to the project. But if not, we have 2 approaches to do that, one is by add reference which will generate an interop dll to convert COM component to .Net component, and the other is by PInvoke to call the methods inside that dll. For uEye_tools.dll, I think you could try to use P/Invoke first, if you still could not work it out after trying, then I would recommend you to contact the third party side for more efficient responses. In addition, you are not able to use regsvr32 to register that dll since it is only adapted for COM components.
For more information about P/Invoke, please kindly refer to these links.
What is Platform Invoke Service
http://social.msdn.microsoft.com/Forums/en-US/vbgeneral/thread/ea96d60f-c468-493c-b145-39af98aa6ddc
Using P/Invoke to Call Unmanaged APIs from Your Managed Classes
http://www.microsoft.com/indonesia/msdn/pinvoke.aspx
Calling Win32 DLLs in c# with P/Invoke
http://msdn.microsoft.com/en-us/magazine/cc164123.aspx