Put a hidden control on your form. When the user enters a month into the field use code to enter the next month in the hidden control (you have to use code because if the user enters 12 you don't want to put 13 in the hidden field but 1. And becareful about the year as the example here will crosses a year boundry). Now in the criteria line of your query use code like this:
Between [Forms]![NameOfForm]![ControlUserEnteredDateInto] And [Forms]![NameOfForm]![HiddenControlWithNextMonth]
I don't know how you have your field set up in your query but you can use the Month() function to get the month number value of a date in your query (2 = February, etc.).
hth,
Jack