I had the same issue. This is how it started working for me (I have the ML629).
Need to state the obvious:
You must install the MaxiSuite software for your computer (Win, Mac, etc.)
I have already ran an OBD-II scan on my vehicle and saved the codes.
1. Connect the device (mine is ML629) to the computer using the USB cable provided.
2. Device is powered by USB
3. On the Device, navigate to Playback and press OK button.
4. Select the data you want to Review (Press OK button)
5. Select the code you want to view/print and press OK button.
6. AT THIS POINT, start you PC Link program on your computer.
takes about 2-3 seconds and then you should see the message change from
"No scan tool connected!" to "The scan tool already connected!"
once you navigate from the Main screen and into Playback is when it
recognizes the device plugged in. You can see this by navigating in and
out of the Playback menu and the PC Link screen will change the message
back and forth.
I hope this helps.