Setting Parameter Data Type/Properties

I have a script which I've added to a toolbox in ArcGIS 10.0. The script asks the user for a geodatabase (I'm creating and storing rasters from a NetCDF file) and either uses an existing geodatabase or creates a new one if it doesn't exist. In adding the script to a toolbox the problem I'm having is what parameter data type to select and/or what parameter properties need to be set to allow me to put in a geodatabase that doesn't exist.

The script runs if an existing geodatabase is selected but not if a new one is selected.

Thanks,
Mike