Unzip the downloaded lcp folder.
Open Rawtherapee and import your photos.
Open the photo you want to correct.
On the right you'll see a column headed processing profiles. Go to the second row down and click on the symbol that looks like a triangle with a pair of scissors.
Scoll down to 'Profiled lens correction'.
Click on the folder to the right of the heading 'LCP File'.
Navigate to the lcp profile you want to use (probably in 'Downloads'). Click on it. The lcp should immediately correct your photo. If not, click on the 'distortion correction' button in Rawtherapee.
Save your image
Hope this works for you!