Answer: Vendors and customers are added in the Company screen.  You can access the Company screen via Administration / Companies.  A single company record can actually be a customer, vendor or both or neither.

Once in the Company screen, you can insert a company like you insert a record in any other screen.  The screen is setup as a master / detail form where you enter the company info at the top and then one or more addresses for that company at the bottom.  When entering the address at the bottom, be sure to click the Vendor and / or Customer checkbox.  Post the record and it will show up in your pick list of vendors if you check Vendor and Customers if you checked Customer.

Keep in mind that you must enter at least one address record for the company.  You must check Vendor or Customer in order for the record to show up in the applicable selection list.  See the sample screen shot below for an example.  Additional information is available through the application help.