I just verified in a sample project that it works correctly.
You should NOT need to call ProcessImports - the less preprocessor should do this automatically. ProcessImports is for @imports in standard CSS, which aren't processed by default.
I suspect what happened is that NuGet didn't add the file that registers the preprocessor. As a result the less preprocessor is never called. If you look under App_Start you should see a file called SquishItLess.cs with the following contents:
[assembly: WebActivator.PreApplicationStartMethod(typeof(MyProject.App_Start.SquishItLess), "Start")]
namespace MyProject.App_Start
{
using SquishIt.Framework;
using SquishIt.Less;
public class SquishItLess
{
public static void Start()
{
Bundle.RegisterStylePreprocessor(new LessPreprocessor());
}
}
}
If this file is missing, you can either add it or add the Bundle.RegisterStylePreprocessor line in your Global.asax.cs' Application_Start method.
If you're installing to a VB project this is a known issue (https://github.com/jetheredge/SquishIt/issues/232) and will be addressed when the plug is pulled on .net 3.5 support.