Gonna copy from a previous comment I posted
It’s not obvious where you are supposed to login; I struggled to find it at first too. Open your hamburger menu on the left, and at the top you should see a little arrow pointing down. Click it, and a new button should appear called “Add Account”, and click it.
There, you can select what Lemmy instance you want to login with, then put your E-mail and password.
I feel like Jerboa for Lemmy (Android’s Lemmy App) could benefit from some of these suggestions as well, so hopefully the dev is also taking a look at this thread xd.