Budget can be found here:
To add a new budget, click on the + next to budget:
You can then enter a name and an estimate for the budget:
You can now save your budget. You are ready to use it.
When you assign a resource to a task, at that point you can link it to a designated budget. On every new assignment, "assigned", "actuals" and "estimate to completion" values will be recalculated for the budget to display up to date information.