In order to localize a .NET application each form or user control of the application has to have Localizable property set true. This makes the IDE to store the properties of the form in the .resx file instead hard coding them to the source code. Having the resources in the .resx file makes localization possible.
To change the Localizable property true open your project, select each form and user control and make sure that Localizable property is set to true.
If you localize a project or solution file (e.g. .csproj or .sln) save your application project and rescan your Sisulizer project. If you localize an assembly file (e.g. .exe) recompile your application and then rescan your Sisulizer project.
Read more about .NET localization.
You can turn this warning off. Choose "Tools | General..." and select "Output Messages" sheet.