To mark a task as completed in Entourage, just check the box.
Clean Up Your Lists
If you mark tasks as completed instead of deleting them, periodically archive or delete old tasks to keep your lists trim.
To delete the task instead, press Fn+Delete on a Mac with the task highlighted in the list.
CAUTION! Project – Next Step tasks are special. Re-process them when you see them, do NOT mark it as completed until you have re-processed it or the entire project is complete!