There are 2 ways to add Contacts on your account:
If you want to add a call or text located on your inbox to your Contacts, just click on it to expand it, and then click on the 'More' button:
Then click on 'Contact':
The default caller information will show. Click on 'Edit' to modify it:
Fill in or edit all fields that you want to use, and hit Save when done.
You can manually add a Contact even if it doesn't show on your inbox. Just head over to your 'Contacts' page on the left menu:
Then just click on 'Add New Contact' button:
Again, just fill in the fields that you wish to have for this Contact. All information will get stored for future usage.
Remember ! If you already have a Contact list, check out this article to import them to your account.