Company management typically designates various individuals to be mentors because of their ability to help, and not because of their ability to hide. Management would most likely be quite put off by a mentor who puts employees off.
Since you have made several attempts to meet with your mentor, but you have only met resistance, you should meet with your manager instead. In such a meeting, rather than making any disparaging remarks about the mentor, simply indicate that you believe that he is too busy to give you the help, guidance, and direction that you seek, adding that you believe it is not fair to him to have this responsibility.
With this approach, you certainly get your message across, but you are doing so in a positive, supportive, and constructive way. You should also ask your manager if there is anything you should be doing to help build the working relationship with your mentor, and be sure to follow any of these suggestions.
If time passes and your mentor is still invisible, then it is apparent that your company's mentoring program is invisible too. In such a case, your mentor is actually giving you some very valuable information about the company.
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