You should be able to open it with a screw driver and pull out the filter for cleaning purposes. Depending on the age of the unit this compartment may have gotten corroded shut. If that is the case, it is probably best to go out and buy a new AC unit so that you don't keep having issues.
Likely, you must look to the bottom of the vented cover and you will find two recessed phillips head screws. Remove these screws and the cover will pitch away from the unit.