To login to Plesk go to:

https://your.ip.address.here:8443/

Login: admin
Password: Chosen during the initial setup process

If you are logging into a freshly installed Plesk control panel, then you will need to use the default credentials:

Login: admin
Password: setup

During the initial configuration, you will be prompted to choose your own password.

Useful Links:

How to reset your Plesk password
How to view your Plesk password