If you haven't got type information, and the compiler can't infer a type then there will be a problem.
Although you are getting output from the command line I would still expect it to give an error. Web Essentials enforces things more strongly, which is the idea behind the tooling.
If you can't get type information (check Definitely Typed on GitHub) you can fix the error using a simplified definition...
interface JQueryStatic {
validator: any;
}
If you have a definition for validator, you could use this trick to declare unobtrusive against that, which would then only have missing type information rather than all of validator.
This will lose typing on validator but will fix the error.
Taken from http://www.stevefenton.co.uk/Content/Blog/Date/201301/Blog/Complex-TypeScript-Definitions-Made-Easy/