First click on Mfr Settings toolbar at the bottom of the screen.
Then find your supplier, by scrolling down or typing their name into the search bar.
Type the percentage you receive off list price into the discount column.
You can also set your Preferred Suppliers which will aid your searching by promoting them to the top of the catalogue search order.
To do this use the drop down in the Mfr Status column and select "Preferred/Buying Group"