There are two ways to import lists you already have email addresses for.
The first way is to click Leads
Within Leads, click Import List
Within Import Contacts, click Browse to find the CSV lead list you're looking to import
After selecting the list, click Next
After clicking Next, the Map Properties section will appear where you will be able to view all data fields within your list. After ensuring that all fields from your CSV match up with the drop down menus, click Next
Once you've clicked Next, this will bring you to the import contacts page. After double checking your contact format and imported properties, click Import Contacts
The second way to import contacts is to click Leads
To complete this process, follow the steps from above.