First, trap the fields that the user enters into variables, then do a find for a date range...
Set Variable [$startdate ; Value: StartDateField ]
Set Variable [$enddate ; Value: EndDateField ]
Enter Find Mode
SetField [YourDateField ; $StartDate & "..." & $enddate]
The first two trap the values entered by your user
Then in Find mode, you set the criteria you are looking for.
The thing I think you are looking for is the "..." which tells FMP that you are looking for a date RANGE.
Edit: Note that if you are using global storage for your user fields, the global fields are available to you in Find mode. This means that your script could skip the variable trapping and simply use:
SetField [YourDateField ; StartDateField & "..." & EndDateField]
but this would ONLY work if your two fields use global storage.