Use "the" when you are talking about the well-known platform as a specific entity. In Russian Russian does not use articles (like "a" or "the").

"Скачать программу rutor.org" (Skachat programmu rutor.org).

If you are trying to make the phrase sound more natural in a Russian sentence, you might add a demonstrative pronoun like эту (this) if you've already mentioned it, but it is not grammatically required.

You would use the definite article because you are referring to a specific, unique website or its specific software client. Correct: "How to download the rutor.org program."

In the phrase (Russian: скачать программу rutor org ), the proper article to use depends on the language of your text: In English