Plagiarism of former supervisors copying from my master thesis
I can accuse my former supervisors of plagiarism, contacting the editor of the journal and proving that a very large portion of the paper is taken from my thesis, which is not cited. Although my thesis is not available online, the authors could have included it in the list of references, and I can prove that they have a copy of it;


Yes, you can do that.


  I can ask to be included as author of the paper, since most of the work published is based on my master thesis, and not only the "simulations", as the authors wrote in the acknowledgements.


Yes, you can also do that.  Based on the information in the question, you should be an author on the paper.  I suggest you contact your supervisors and ask them to fix the author list first.  If that is unsuccessful, you should follow your university's grievance procedure.  Contacting the editor should be the last resort.