It looks like OBDotNet hasn't received any attention for quite some time.
I don't have the appropriate compilers, so it's really hard to make / update those packages. It's pretty easy to update, though. Make sure you have SWIG installed and then when building Open Babel, make sure CSHARP_BINDINGS is set to true.
If someone is willing to provide updated DLLs, I'd be happy to make sure they get posted / updated.