Company in Odoo has two meanings: 

1. An organization - with a set of partners (leads, customers, suppliers and employees), users (logins) and stock of products with a separately managed set of accounts.  This type of company installs Odoo!
   
   

2. A partner that isn't a person - a 'type' of Customer or Supplier to support B2B (as opposed to B2C) transactions. An entity that buys from (or sells to) a type (1) Company. This type of company is sent emails and PDF's from Odoo!

Companies (type 1) are imported via

• Settings --> Companies --> Companies

Partners with "Company = TRUE" (type 2) are import via:

• Messaging --> Contacts (default CUSTOMER=True, COMPANY=False unless overridden in your import file) 

• Sales --> Sales --> Customers (default CUSTOMER=True, COMPANY=False unless overridden in your import file)

• Purchases --> Purchase --> Suppliers (default SUPPLIER=True, COMPANY= False unless overridden in your import file)

To your question - which menu are you at when you are trying to import?  You won't see Supplier and/or Customer as an option for type (1).