Your WHM login details can be found in your New Account Information email, or in your Pixeno client area under your hosting tab (these will show the login details set initially, and not updated ones if you have since changed the password).
To add a new website, on the left hand side (once logged into WHM) - search for: Create a New Account. You can proceed with filling out the form for the domain name, username, password and contact email address for cPanel hosting account you are adding. You will also need to define what package or resources you would like to give the domain here also.
You will need to set your domain names nameservers at your existing domain registrar to the following:
Please bare in mind this can take up to 24 hours to take effect, due to DNS caching on your domain name with internet service providers.