I think you do have justification for feeling disappointed, though given the fact that the shiur was free as a public service, and the possibility that there may have been a miscommunication between the organizers and the presenter, I think you are also correct that you probably shouldn’t say anything.
I feel that the reason you may question whether you have the right to feel disappointed comes because what the shul is doing, offering a free shiur to the whole community plus dinner, is a very nice, upright thing to do. That factor acts as an emotional counterweight to your disappointment.
So here’s a parallel situation. Suppose I advertised in a paper that I wanted to give away a certain rare item for free . You have been interested in acquiring that very item for some time, so you make a long schlep over to my place to get the item (let’s assume you didn’t call to confirm, but somehow knew the item wasn’t taken yet). When you arrive, I say that I’ve changed my mind and want to keep the item, but here’s a different item for free that has the same monetary value. Would you feel disappointed? Would you have the right to? After all, your time was spent in getting there.
In the above case, I wouldn’t think “damages” should be paid, but a statement of “I came a long way and unfortunately this wasn’t the item I was expecting” followed by an apology to you would seemingly be justified. After all, it was my advertisement that was responsible for bringing you out.