Translation:I know how to use a computer. In addition, I am an accountant.
"I know how to use computers" should be accepted. The sentence is talking about knowing how to use computers in general. If the object of the sentence was "the computer", it should be "Tôi biết dùng cái máy tính". If it was "a computer, it would be "Tôi biết dùng một máy tính".
Also, I think "also" should be interchangeable with "in addition", no?
"I know how to use computers" should be accepted. It's the correct translation. In this case 'tôi biết dùng máy tinh' ís a generalisation. 'Computer' is a countable noun. Generalisations of countable nouns in English take a plural form. E.g. 'I like dogs'. Also, "furthermore" should be accepted.