You can set the order before you create the Portfolio by dragging them into place (this works in list and thumbnail view):

After you've created the portfolio, when viewing the portfolio you can change the order by:
- Right-clicking in the Names list on the left, and choosing Sort by order

- or Choose View > Portfolio > Details, the click on one of the headers. You can edit the values of the Description and Name (Right-click : Edit Value > Name or Description.
Also you can add your own order if you want:
- Right-click on files list and choose Portfolio Properties
- Click Add
- Name the field.
- Click OK

- Now in Details view (View > Portfolio > Details), you can edit the values for your added field. Right-click and choose Edit Value > myOrder (as per example above).

- Enter a value and click OK, you can now sort in ascending or descending order by clicking on the header for the new field, or if you are in layout view (View > Portfolio > Layout), right click in names list and choose Sort by.