Step 1: Understanding the Concept:
Selecting the appropriate conditional subordinating conjunction expressing negative condition.
Step 2: Detailed Explanation:
The conjunction 'unless' means 'except if' or 'if... not'.
The sentence conveys that computers provide no utility except under the condition that one possesses the knowledge to operate them ('Computers are useless unless you know how to use them').
Step 3: Final Answer:
Therefore, 'unless' is the correct conjunction.