Make sure you come to a complete stop before trying to change gears. Also, make sure that you feel the detent on the shifter and see that the cluster display shows the correct gear selected.
There's a tip on our FAQ (http://www.smartusainsider.com/faq#faq05).
Let us know if that helps.
This post is nearly 5 years old. Anyone tried to get smart USA to cover this problem recently? My '08 cabriolet has 60K miles and started having this issue several months ago - often won't shift into reverse and sometimes drive when cold but don't think it has been an issue yet after it warms up.