The hood release is on the driver side footwell, mounted to the outside. If pulling the handle doesn't release the hood, you could have a snapped hood release cable, or you could have a worn sheath around the cable.
You can remove the hood release lever by taking out the two phillips head screws that hold the frame in place. Using a pair of pliers, pull on the cable that's exposed. If the hood releases, you likely have a worn sheath. If it doesn't (and the cable pulls WAY out) you have a snapped cable.
You can do a temporary fix for the worn sheath by adding a spacer. Look at the hood release handle, where the cable slots into it. There's a recessed area that holds the sheath still and provides leverage to pull the cable. Take a small (not more than 1/4" ) piece of vacuum hose, or some other piece of hose, and slot it into place in that recess. That will pull the sheath out far enough that the hood release will work. The long-term solution, of course, is to get a new cable. Try your local MB dealership. Some of their pricing can be scary, but some of it is shockingly reasonable. The only way to know is to have them price it for you.
And, FYI, most dealerships give a 10% discount to any customer who is an active member of the Mercedes Benz Club of America.