What is the configuration that should have my add-on projects?
The .NET framework version used by E-Sniper is the 4.0, so you can develop your add-ons with VisualStudio 2010 or 2012.
E-Sniper is compiled with the x86 platform option, so that it can run on computers with 32-bit operating system. In addition, the library with which the USB cameras are managed is a 32 bits assembly. Therefore, the E-Sniper add-on projects also must have this platform as destination, not being valid x64 nor AnyCPU options, because this generates x64 code in 64 bit operating systems.
Also you should always add a reference to the assembly E-Sniper.AddOns.dll, that contains the core of the SDK. This library is always distributed with E-Sniper, so you should not include it in your list of files or the installation will fail, because the program does not allow overwrite any system assemblies for safety.