.rar files are folders, kinda like .zip files.
1. Get WinRAR- there are free versions out there but there is also a trial on the site.
2. Open the .rar file in WinRAR- it will show you whats in that folder, like a .psd, .jpg, .png of the thing you need for gimp.
3.
A. Extract the files in the folder- there should be an Extract Files button, click that then put the location of where you want the files extracted, the extract.
B. Click on the file through WinRAR- it will open up in GIMP.
If you choosed A then go to the place where you extracted the files and click on it to open it. Also by choosing A, you will always haved the file saved if needed.