One of the keys to my ML320 failed (not just a bad battery.)

I purchased a new key from a Mercedes dealer by mail and it arrived
today, properly cut so that it operates the ignition lock.  What it
doesn't do is lock or unlock the doors.

I've tried the procedure in the Operator's Manual to resynch the key
without success.

It's a 100+ mile drive to the nearest Mercedes dealer so I'd like to
avoid that option if it is something I can do here.

Any suggestions?

Boden