If you are using IIS6 or not using IIS's integrated pipeline, you'll need to configure your web server's settings so that requests for .LESS files are routed to the ASP.NET engine (so that they can be processed by the .LESS HTTP Handler):

1. In IIS Manager, select the website you want to configure for .LESS and on the Actions menu, select Properties.
2. On the Home Directory tab, click Configuration.
3. In the Application extensions list, click Add.
4. Fill out the fields as follows:
   • Executable: path to the ASP.NET DLL, by default: c:\windows\microsoft.net\framework\v4.0.30319\aspnet_isapi.dll
   • Extension: .less
5. Uncheck the Verify that file exist option.
6. Click OK in the Add/Edit Application Extension Mapping window, and then in the Application Configuration window.

Now add the MIME Type for .LESS in the website's properties:

1. On the HTTP Headers tab, click MIME Types.
2. Click Newand fill out the fields as follows:
   • Extension: .less
   • MIME type: text/css
3. Click OK in the MIME Type window and then in the MIME Types window.
4. Click OK in the Properties window.

Also, make sure that dotLess handler is defined in the <system.web> section in Web.config:

<system.web> 
  <httpHandlers> 
    <add type="dotless.Core.LessCssHttpHandler, dotless.Core" path="*.LESS" verb="*" /> 
  </httpHandlers> 
</system.web>