In order to use the Manage Budget feature, you must enter Supply Pricing (see related article)
Go to Functions > use the search box or 'Advanced Search' to find the Function you want to enter a Budget for
Click on the Function
Click on the 'Manage Budget' button
Click 'Add New Budget Item'
Enter required fields, including the 'Detail Type', and click 'Save'
Once all Supply Pricing and Budget Items are entered, you can run the Budget Report (see related article)