To access the budget, open a project and select Budget from the left hand menu bar.
Note that the budget will only be available on projects where you are using the estimates & job costing features of CoConstruct.
If you're running an open book or cost plus project, this will automatically be enabled.
Otherwise, to turn estimates and job costing on or off, head to the Project Setup page and look for the "Estimates & Job Costing" option.