Salsa and Merengue sound so alike, I can’t tell the difference between them.
I agree with most of what Jay B. said except that she could have chosen a better Merengue dance video as an example for you. If I tried to dance like the show performers in that video, I might injure myself or someone else… then I would be making an appointment to visit your office.
You may want to buy a couple of Merengue CD’s and a couple of Salsa CD’s and listen for the differences. You might want to shop for CD’s that give you a mix of several different artists on one CD.
Since you want to know the basic rhythm of Merengue I suggest the following video for the music: http://www.youtube.com/watch?v=0x9gesXt8yI
The Salsa instructional dance video that Jay B. recommended is a good one.
The following Salsa music video I selected is a bit fast tempo, but it will help you to hear the difference in the rhythm and melody between Salsa y Merengue: