If you're entering 3 for March, then they probably set it up as a number, and the year as a number, but whichever it is, the record selection would be something like:
{table.date} >= cdate({?MyYearParameter},{?MyMonthParameter},1)
and
{table.date} < dateserial{?MyYearParameter},{?MyMonthParameter}+1,1)
If they haven't set it up as a numeric, you should. You can also manually populate the month names in a pull down list of a numeric parameter to display ONLY the description, so that the users see the months, but you're working with a numeric.
Easier to code, and better for the user to just select a month.
-k