Translate next post/previous post and search results for

Hi, I have the Catch Fullscreen Pro theme. My website is https://www.duneart.nl The navigation labels "Next and previous post and Search results for" are not translated to Dutch. I tried to work with the nl-NL po file with poedit free software but I do not know how to add the translation strings to ths file. Can you please advise me how to have those translations done in my theme? Many Thank's Dirk