Email joao.frasco@yahoo.com. There is no separate account to sign in to. Your logs stay on your device and in your private iCloud.
Include your device model, iOS version, the app version, what you expected, and the steps that reproduce the problem. Do not send passwords or payment card details.
Logging stays free for about 12 things. When Last? Pro adds unlimited things, photos on entries, themes, app icons, and extra community slices. Restore Purchases is on Settings → Upgrade to Pro. Refunds go through Apple’s Report a Problem.