To add expenses:

From the main menu, click Add > Expense.
On the pop-up window, select either of the options: In-Progress Task OR Other.
The In Progress option will show you tasks you have recently visited.
The Other option, will let you select the Project and Task from the respective drop-down list.
Click Next.
In the Summary box, type the summary of the expense.
Click Add Line Item to enter the expense details.
On the new pop-up, under the Expense Code drop-down list, select an expense code.
In the Description box, enter the description of the expense.
In the Amount box, enter the amount.
Select a date from the Date picker on which the expense was incurred.
If this expense item is reimbursable, then check the Reimbursable option.
Click Save.
On the main page, select the Billable or Costable option, if you want the expense to be billed to the client or incurred as cost.
Click Save. The expenses are added to that project.

The Costable or Billable option is remembered as a user preference i.e when you select this when submitting an expense, it will be remembered when entering expenses in future.