To add an expense, go to Accounting → Expenses → Add Expense. From here, you can select the desired currency, amount, details, client, project, set a tag if you wish to filter after it later on or even add a receipt.
The expense will be reflected at a project level in the Overview tab → Unbilled AR plug-in. Just hover over the question mark icon for it to show up
NOTE: The expense has to be in the same currency as the project for it to show up under the Unbilled AR plug-in.