Where to get help when we can't understand something done in a paper?
Assuming that you've exhausted a reasonable degree of effort to try and understand what the author is doing, you can always just contact the author directly. There is usually contact information on the paper for the corresponding author. 

There have been a number of papers that I have read and needed to reach out to the authors for to get a better understanding of what is going on in their paper. Professors are people too. As long as you have demonstrated a reasonable degree of effort prior to reaching out, asking for some clarification from them is a reasonable thing to do. 

In the case of an author having passed away... then you will want to communicate this to your supervisor and explain why you need help to understand the paper. If you cannot figure it out for yourself, do not have any peers you can talk to, or are unable to contact the original author, then you have your supervisor as a work colleague.  

If that all fails, then you will just have to try and drudge through understanding the computation. While I understand you want the answer quickly, all things worth while do not come quickly, and you'll need to cultivate and demonstrate resilience in the face of that struggle.