On mine it's just the 4 screws (T-20?), 2 on each side.
Then it pulls straight back, disengaging the front "fingers" that interlock to the underside of front bumper.
And just to clarify, the plastic piece is a Splash Guard, it does virtually nothing to prevent oil pan damage if you hit something.
If you truly want a Skid Plate- here's the real deal:
Panzer Plate heavy duty aluminum skid plate kit for 19982010 New Beetles by www.dieselgeek.com
I fear speed bumps no more !