To find the login it depends on which page you are. There are three ways to find the login.
First you can directly access the login page using this link: https://www.yumpu.com/en/user/login
If you are on the front page you get two version depending on where you are located. Most of you will see the login at the top right.
Alternatively you can click on "Register". On the registartion page you will see the "Login" button at the top right.
For those from the UK, you will see a "profil icon" with click on that you can choose "Login to YumpuPublishing" to get to the used login.
If you forgot your password, you can learn how to reset your password here.