Sounds like you have a good case for a copyright infringement lawsuit. The first step could be a cease and desist letter with a demand for compensation. See a lawyer who handles copyright infringement cases right away.
I always attempt to provide pertinent and relevant information. However, intellectual property law is complex and cannot be fully explained in a brief communication like this.
The concept and ideas for an expo are generally not protected. Copyright, however, may protect the written marketing materials you created -- as long as you were the original author (with proof) and you can show that the materials in use by the new expo creators are substantially similar to (or, preferably, verbatim copies of) your original materials. You should consult with an experienced copyright litigator ASAP, and you may need to look outside Ohio. The Copyright Act has very strong remedies, but you need a thorough analysis from someone with experience before going any further.
This comment is NOT LEGAL ADVICE and is posted for informational purposes only. I am not your attorney; you are not my client. Both you and any other person reading this comment SHOULD NOT RELY UPON this comment. Regardless of the information provided in this comment, any reader of this comment should CONSULT AN ATTORNEY to get the legal advice you are seeking.
Ideas, concepts, etc. are NOT protectible in the absence of a contract such as a non-disclosure agreement or implied contract for compensation. But the EXPRESSIONS of ideas, whether registered for copyright or not, may be protectible under copyright law if those expressions are novel enough, and the bar for hos much originality one needs to qualify for a copyright os quite low.
Here's your biggest problem: not having these expressions registered for copyright means you wouldn't be entitled to STATUTORY damages or your legal fees, just your ACTUAL damages. And what would those be? How much of their expo's income is based on "your" materials?
You need to see your own copyright litigator ASAP to explore the details of your situation.
Avvo doesn't pay us for these responses, and I'm not your lawyer just because I answer this question or respond to any follow-up comments. If you want to hire me, please contact me. Otherwise, please don't expect a further response. We need an actual written agreement to form an attorney-client relationship. I'm only licensed in CA and you shouldn't rely on this answer, since each state has different laws, each situation is fact specific, and it's impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue.
It does sound like you have a valid copyright claim. You can possibly obtain an injunction to stop them from using any materials at the expo. The "compensation" you could potentially receive would be included in the damages you can recover in the lawsuit.
I doubt that your claims are worth very much. Our intellectual property laws do not protect ideas---they protect tangible inventions, or works of authorship, art, music, film, theater, painting, books etc. You might have some claims for copyright infringement, but you can't bring those claims unless you have registered your works with the U.S. copyright office---and nothing in your question indicates that did so. Since you did not register within three months of publication or your original works, you have given up your right to pursue statutory damages and attorneys fees, and you left with a weak case where you would seek profits attributable to the specific copyrighted works---but this would probably be only a small portion of the profits. Further, you might be able to claim lost profits, but you would have to show they are attributable to the specific copyrighted works rather than your general idea. In short, these cases seldom result in any significant financial recovery---but if you want to invest the resources as a matter of principle, please hire one of us. Our fees will probably be higher than the value of the case but justice might prevail,