If you want to use a separate admin project, you need to copy the files Settings.cfg and InstalledPlugins.cfg from the AppData directory of the Grand.Web project to the Grand.Web.Admin project. Please also remember to copy the plugins folder.
But there is still an issue : I don't have any InstalledPlugins.cfg file in AppData directory of the Grand.Web project or anywhere in the solution. So when I want to add plugins, the Grand.Web.Admin project doesn't start anymore...