You can manage your billable hours for a specific project directly from the Project Status view, or from a Project report: click on the number of hours in the "Budget" section and then click on Billable Hours:
- Select the filters. You should see the checkbox appear next to the entries;
- Start selecting the entries: you should see the Billed dialog appear;
- When you're done selecting the entries, click on Billed from the drop-down menu.
If you need to mark entries as billable or billed on the fly, you can do directly from the Entries log.
Simply open your entries log, click on the filters and start selecting the entries.
Then you'll be able to mark the entries (or a copy of the entries) as billable or billed.