The town of Isla Mujeres [1] (known as the centro, or center) is at the far northwestern tip of the island; at just eight blocks long and five blocks deep, it is very walkable.
This is where most of the hotels, restaurants, shops, and services are. There is no main street per se, although Avenida Hidalgo intersects with the town zócalo (central plaza) and has a bustling pedestrian-only section. Avenida Rueda Medina is the busy street that runs along the south side of the centro past the ferry piers and continues all the way to the island’s other end, becoming Carretera Punta Sur at Parque Garrafón [2] (and therefore also known as Carretera Garrafón). The road that runs along the north side of the island is Avenida Martínez Ross as it leaves the downtown area, becoming Carretera Perimetral partway down the island.
Links:
[1] http://www.moon.com/destinations/cancun-the-yucatan/cancun-cozumel-and-the-riviera-maya/cancun/isla-mujeres
[2] http://www.moon.com/destinations/cancun-the-yucatan/cancun-cozumel-and-the-riviera-maya/cancun/isla-mujeres/sports-and-recreation/parque-garrafon-ec