Indicating the product's supplier is not really important to your customers (much less so than its manufacturer in any case), but it may turn out to be an essential part of your own internal management, not the least when managing your stock: you simply need to know who you bought the product from. The supplier of the current product is to be set from the "Suppliers" tab on the left.
You cannot use this feature if you do not already have at least one supplier registered in your shop.
Associating the current product with one or more suppliers is really easy: simply click the Options tab and go down to the Supplier's zone. You then get a complete list of all of the suppliers you have created. Just check the box corresponding to the supplier, and then save your changes.
If the product is associated with more than one supplier, you can choose which one should be the default one using the dropdown menu under Default Supplier. Just choose the supplier you want to define.
The product's supplier page also features a table that enables you to set the precise reference and unit price/currency for each product combination. If the product has more than one supplier, the table only opens the combination tied to the first supplier, and the others are closed by default. Click on a supplier's name in order to open its references table, and close the others.