A sign-in activity email doesn’t always mean someone else accessed your account. You might get one even after signing in yourself. Use the details in the email (When, Device, Location and IP address) to check. There’s no sign-in history or list of connected devices in MYOB Acumatica.
A genuine sign-in activity email comes from no-reply@myob.com and does not include a link to sign in or enter a password.
What you need to do
If you were the one who signed in, you do not need to do anything.
If you do not recognise the sign-in, reset your password immediately.