I believe WenWe is on the money here. People who work "for free" are actually gaining experience and potentially getting a foot in the door, which is likely to be considerably higher value than apprentice wages. In turn the employer is footing a not inconsiderable bill for supervision, instruction, clothing, passes, etc. Remember trainees can't work unsupervised so not only does an experienced man have to shadow their every move but in explaining and demonstrating what has to be done he won't be properly productive himself.
I applaud any company that takes on trainees these days. They are the future of the industry but with the importance of engineering skills in what we do employers need to be sure their time and money isn't wasted on somebody without the ability and will to stay and do a good job after training. Hence they are selective and the only way to select is to see how somebody does in the working environment.
Saying that I wouldn't expect an employer to have somebody working for free past an initial trial period. If they are going to commit to a trainee they should expect the trainee to commit to them and as part of that agreement pay the person.
To the OP, study the requirements for the job, either from an advert (if there is one) or generically. Then write your CV to emphasise how you meet those requirements. You will only get 10 seconds of a reader's time so make sure it's well laid out and clearly shows the salient points.
Remember, you wouldn't use the same chat-up line for every girl, don't expect the same CV to work for every employer!