OK
Not 100% how you are setup -- but you need to go to the chart of accounts and setup an account in the 6300's for Travel expenses. If your client needs details by type of expense (i.e., Air travel, hotel, rental car, etc.) you may want to setup one account for Travel Expenses and then detail each out on the invoice and charge it all to this account.
Secondly, in the "items" you can setup an item called expenses. For my consulting business I have two basic items -- consulting hours and expenses. Once you have this item, you can charge in on the invoice.
THe rest should be self-explanatory. Make sure your expenses get charged to the Travel Expenses account. You will do this once you apply the customer;s payment. Make sure you apply the expenses portion to the expense account you setup in the chart of accounts.
Hope this information is useful.