It is not really a theme park. It is called Hezbollah-land as a joke by the press. No rides. This is not Disney.
Here is their website: http://www.mleeta.com/eng/materials/landmark.html
Mleeta, or the Tourist Landmark of the Resistance, as they call it, is worth the trip! I went for the second time in March of this year. The admission cost is a few bucks. The first thing you will do is watch a 15 minute propaganda video and then an hour or so guided tour -- in Arabic. They will call in an English speaking guide, for free, if you give them a few days notice. We received a private tour at no additional cost.
I have a great driver in Lebanon. Feel free to PM me if you want his info. He is from the Baalbek region but now lives in Beirut. My buddy and I got the impression that he is a Hezbollah sympathizer, but didn't ask to confirm. The cost for a private driver from Beirut, roundtrip, is around $60-$80, including a few hours of waiting and lunch.
You will want 2-3 hours for Mletta. Some drivers have a hard time finding it, so it is best to go with someone who knows. Always ask for Mleeta or the Tourist Landmark of the Resistance.