This type of drip pan under your freezer is not intended for removal. You may be able to gain better access to it by unplugging the freezer and carefully moving it away from the wall. I recommend using a wet/dry shop vacuum to reach into the back of the freezer and suck out the water from the drip pan if it is filled. You may need to use an attachment to reach the drip pan. Be careful not to disturb the sealed system tubing in the bottom of the freezer. After vacuuming out the drip tray, you can try squirting some liquid chlorine bleach into the tray and letting it set for at least 10 minutes or so. This will kill any mildew and may also help to kill the smell. Vacuum the chlorine bleach out and reassemble the freezer. This may take care of the odor problem in the drip pan of your freezer.