Just got my M-pass set up in Pattaya. Notes as of today:
1) Bought it at big building next to the last toll booth at the end of highway 7. It is possible to get there without entering the highway. https://maps.app.goo.gl/MmjoQkEYwwPZ2Uar9
2) Office is on the second floor, open standard hours (Mon-Fri 9am - 4.30 pm I think).
3) Brought my passport and blue book, they also asked for the address in Pattaya. Took 300 baht which was added to the balance.
4) They gave me the box with the device and debit visa connected to the account.
How to check the balance and top up:
5) Register at www.thaim-pass.com -> "Check balance" -> Register at https://www.thaim-pass.com/custinfo/Register.aspx . It will ask for 13-digit tag (printed on the device itself), 10-digit account number (printed on the box), ID (passport) number from step 3.
6) On your phone download M-pass application (google for "m-pass application" or scan one of the QR codes on the box and you'll see the links there). Login to your account using username/password created in step 6. (Note that the app is supposed to let you to register (Step 5) in the app itself but that flow was broken, had to use the web site).
7) App can generate the payment QR code. I was able to scan that code in Kasikorn banking app to top up.