It's easy to assign transactions to a budget item. All you have to do is drag and drop.
1. Click on "Transactions" in the top right corner.
2. You will see a list of transactions that are unassigned. You can then click on the transaction and drag and drop it into the budget item where it belongs.
Once you drop the transaction into the budget item, the "available" amount updates to show how much money is left for that budget item.