It's 'a' link (indefinite article) because it is referring to one specific link that has been found or is being discussed. There may be other links, but we are only interested in this particular one. 'The' (definite article) is used for the other nouns because we are talking more generally about two widespread phenomena - the occurrence of skin cancer and the use of computer monitors. By the way - a still on the subject or articles - can you see the error in your question?

Do these parentheses help you?: "a link between (the occurrence of skin cancer) and (the use of computer monitors)" It is saying that using a computer monitor can make skin cancer occur.