The Keyoti.Thesaurus.Model.dll file must be installed in the GAC, this is done at install time, however it is possible that the assembly was unregistered later.
To add the DLL to your Global Assembly Cache, please open a command prompt and cd into a directory with Keyoti.Thesaurus.Model.dll in it.
Then type:
gacutil.exe /i Keyoti.Thesaurus.Model.dll
If you get an error saying the command wasn't found, you should add the .NET SDK bin directory to your path, eg;
"C:\Program Files\Microsoft.NET\SDK\v1.1\Bin" |