If you haven't already, get a flashlight and see if you can look into the grill to see how much of the handle has broken off. This happened a couple months ago on our '91 300E, and we were able to jury rig a wire coat hanger to open the hood. Unbend the hanger, and then take a pair of pliers to bend the last half inch into a hook to slip between the grill crate and catch onto what's left of the handle. (We were lucky in that the handle broke off only halfway in, as opposed to completely) There should be a slot for the hanger to hook onto where the handle forks to attach to the hood latch. The mechanism of the latch is pretty simple - it's basically a hook, attached to the pull handle, and a lip for the hook to catch. The pull handle just pulls the hook clear of the lip so the hood can open vertically.
This took us awhile to figure out, but most of the work is just maneuvering the coat hanger. If the handle has broken off past the fork, then things get a lot harder...