You can create an expense or write a check to enter payment made for your supplier. Let me show you how.
Go to the +New button.
Select Expense or Cheque.
Under Payee, select your supplier.
Choose an appropriate payment account.
Enter the amount of purchase.
Click Save and close.
If you are wanting to add the payment to a bill at a later date(you paid but are waiting for the bill) you can go to category details on the expense and choose the creditors as the category on the line of the transaction still selecting the supplier in question and then fill in date/amount etc. Or you can do a journal entry if preferred. Then when you create bill and go to pay it you will be able to select the expense to pay the bill.