Don't overthink it. Just pick the phone and call them. You'll find that most people are really nice and will speak to you or advise you. Ask them for relevant information, advice etc. Be straightforward and tell them exactly why you're calling. At least, that's what works for me - I'd just tell them the story about my mentor and about me starting on this new venture etc.
Oh, almost forgot. If they can't help you or are unwilling to speak to you, ask them if they could refer you to anyone who might be of some use to you.