The route to Nivati is pretty much straight forward. You need to take the Goa route (NH17) uptil Kudal and take a right towards Nivati from Kudal. The drive from Mumbai to Nivati will approximately take about 8-9 hours.
If you are driving to Nivati from Pune you have the option of either taking the Gaganbawda route from Kolhapur or bypass Kolhapur and take the Kumbharli ghat to meet NH17 at Chiplun. Though Gaganbawda route would be in better condition it is better to by pass Kolhapur if you are not comfortable asking for directions as its slightly confusing to get to the Gaganbawda route through Kolhapur city!
Pune - Satara - Umbraj (35 ms from Satara) - Chiplun - Kankavli - Kudal - Nivati