If I'm not mistaken, you need to sign-up for BMW assist. In Canada it's free for 1 year, 4 in the U.S. Thereafter, it's $199 a year for Canadians, not sure about any other region though.
Once you've signed up for Assist, you should see the option in iDrive, just can't recall where exactly, as it's been sometime since I've looked through those Assist options (includes other stuff as well, like Concierge etc).
2011 Z4 sDrive 30i; Space Grey, Coral Red Kansas, 6MT; Fully optioned & 2011 28i X3 Loaded