In order to go to ERM:
Bus n°27, station Kesklinn, at Kvartali trade centre (in front of Kaubamaja, near the hotel)
After 10AM there are two buses /hour, at xx.13 and xx.43. The same timetable is also valid from ERM.
Just in front of the hotel, there is the Hansakeskuse busstation, for the bus n°7. You must go to the station Killustik, at 6-7 minutes walk from the Museum.
And map: https://goo.gl/jYwScL
Tickets are sold by the driver; if you have a green card from Tallinn, you can use it to buy your ticket.
The most comfortable way to go to the museum is still to take a taxi. The cost is circa 5 € and with 3 or 4 passengers it is even cheaper than the bus.
The electric taxi number is 1918; another number is 1200. And the hotel has undoubtedly other numbers for taxis.