To access your customers please click on the customer tab found on the left side of the screen.
From the customer list, select the customer you wish to assign products to by clicking on the customer NAME.
Click on ASSIGN PRODUCTS found at the top right of the page.
Move the products that you would like to assign to the right.
NOTE: Make sure that the rep that will be invoicing has the same products assigned as the customer so that the field rep can see the products. If the customer has a product assigned and the rep does not, then the rep won't be able to sell the product to the customer.