JR pass activation in Nagano or Matsumoto?

Is it possible to activate JR pass in Nagano or Matsumoto?
Were are offices located?
What are the working hours?

Is it possible to activate JR pass in Nagano or Matsumoto?

What do you mean by "activate JR pass"? If you mean exchange the MCO into actual pass you can do it at Nagano but not in Matsumoto.

http://www.japanrailpass.net/en/exchange.html

Nagano Travel Service Center 10:00-19:00 (Mon- Fri) 10:00-17:30 (Sat, Sun & Hols)

https://www.google.com/maps/place/長野駅びゅうプラザ/@36.6430784,138.188517,20z/data=!4m5!3m4!1s0x601d86928b76f6a7:0x86848009942c63e9!8m2!3d36.6430114!4d138.188719

When you exchange, the staff will ask you what day will you start using the pass. So if you exchange the MCO into the pass somewhere like Narita, Haneda, etc., you can specify the starting date and then start using anywhere you like (like Matsumoto where there are no exchange office).

Hello Aleksander,
In Nagano, you can activate it at the Travel Service Center. Here's a complete list that includes all the JR Exchange offices in Japan: https://www.jrailpass.com/files/exchange_office.pdf. And more information about where to activate your JR Pass: https://www.jrailpass.com/faq/japan-rail-pass-exchange-offices