When you enter your Stock Control Module, you'll see the following:

To Add a stock item, simply press the + ADD STOCK button. You will then get a pop up looking like this:

Simply input all the data you have - Part Number should be for Manufacturers Part Numbers and Alternative Part Number is for suppliers own variations. When you search for a stock item, the system will search both of these boxes. 

Stock Notification Level is there so you can set an alert when you get down to a certain level of that item. An alert will be in the form of an email telling you that you should order more of that item.

When you're done, simply press ADD TO STOCK and that item is now in your Stock Control Module and is searchable from your account and the Job Card.