Yeah, it's not on Melgar.
Without specific directions from the guy who knows where it is, this is what you do: ask a local.
You can always walk up one street and down the next looking for the sign with a guitar on it, but the local would know better.
For the record, Melgar is the name of the street on which all the "recommended" shopping/dining is. It runs north-south along the waterfront and the tourist shopping district is perhaps 10/12 blocks long. Maybe more. Depending on which way you're facing determines whether you turn left or right -- there is only one direction to go when you're on Melgar: inland, or east.
East-west streets are called "Calle" [Spanish for "street"]; north-south streets are called "Avenida" [avenue]. Right smack in the middle of the tourist shopping district on Melgar is their center street which they call "Juarez". The streets/calles are north or south of that [norte o sur].
If you are docking at International Pier, you will be a few miles south of this shopping area, and you will take a cab to town -- generally to "Forum shops", which is at the norte end of this section. If you are docking at Punta Langosta Pier, you will be at the sur end of this district, and you'll have to wind your way through Punta Langosta mall before getting to the street. Yes, they did that on purpose.
Here's a basic map [you can google for others that are better]. Good luck finding it.
http://www.travelnotes.cc/cozumel/links/maproad.php