Diveagar is around 170 km from Mumbai and 200 km from Pune. Most people drive.
By car from Mumbai, take NH66 (the Mumbai-Goa highway) south to Mangaon, then turn west on SH97 to Mhasla and on to Diveagar. The road is reasonable; allow four to five hours including a stop. The last stretch, after Mhasla, is narrow and twists through the hills — slow but pretty.
By car from Pune, the route is through Tamhini Ghat and Mangaon. The ghat road is spectacular in the monsoon and dangerous in heavy rain; in dry months it cuts the drive to about four hours.
By bus, MSRTC runs services from Mumbai (Borivali, Dadar) and Pune to Mhasla, with local connections on to Diveagar. The Mumbai-Diveagar overnight option is grim but cheap. Private operators run Volvo coaches to Mhasla on weekends.
The Ro-Ro ferry from Mandwa to Mumbai (and the catamaran from Gateway of India to Mandwa) lets you skip the worst of the Mumbai traffic if you are coming from the city; from Mandwa it is about three hours by car to Diveagar.
The nearest railhead is Mangaon on the Konkan Railway (Mumbai-Goa line), about 50 km away, with auto-rickshaws and taxis available outside the station.