You could add a rule with an action that sets the value of the field
corresponding to the drop-down list box (the value of this field would have
to correspond to a value that the "Value" field of the drop-down list box
contains) to the default value required. So if you have the following items
in your drop-down list box:
Value - Display Name
1 - Item 1
2 - Item 2
3 - Item 3
and you want "Item 2" to appear as the default, you would have to set the
value of the field on the form that corresponds to your drop-down list box
equal to "2" (=the value of the "Value" field of the drop-down list box) when
creating the action for the rule.
You could let this rule could run e.g. when the form is opened, by adding
the rule through to the "Tools > Form Options..." menu item, clicking on the
"Open and Save" tab, and then clicking on the "Rules..." button to add the
rule.