There is a great club with live Merengue orchestras right in the Juragua! When will you be there?
There is a club right behind the Juragua called Jubilee. The club in a cave is Called Guacara Taina and is very cool.
I think to learn you are better off in a quiter club or going early. If you get to the Merengue bar in the Juragua early, it will be quiet and there will most likely be a few girls that would love to teach you to dance. Buy them some drinks and enjoy.
Another possiblity is the hotel Lina which also has orchestras but is quieter than the Juragua.
If you want to learn/practice Bachatta, there is a place near the airport called Pio Pio that has some Bachatta orhcestras on the weekend.
I'll be down there june 1-6 and if you are going to be there then give me a buzz on my cell phone 809 751 2411, obviously when in the DR there is no need to dial the area code. I'd be glad to help you out with some info.