It is allowable to save a uir file outisde the directory of your project file, and you do not need to use the full pathname of the .uir file in the LoadPanel function.
When you add your .uir file to your project (in the project window: edit -> add files to project) you are specifying where the file exists. Make sure you have not moved your .uir after having added it to the project (in the project window: view -> show full pathnames). Perhaps the pathname shown does not match the actual location of your file.