Shut down the light, wire the meter in series on the (+) driver output. Connect the (+) terminal of the meter to the (+) lead from the driver, then connect the (-) terminal of the multimeter to the (+) input on the LED and finally the (-) output of the LED to the [-] lead from the driver.
When measuring the current, the power needs to flow through the meter in order to get a reading. The internal resistance of the meter is rather high, by design. So if you have a wire leading from the [+] led of the driver to the [+] impute of the LED in addition to the multi-meter, you've effectively created two parallel paths for the power to travel. In which case it's going to choose the path of least resistance and you won't get a proper reading.
Someone else may have some corrections to this. Or better wording.